Ukraine is in slow but insistent meltdown.

In recent weeks the country has endured a violent revolution that included the collapse of its government and seen Crimea, a major swathe of its territory, transferred unceremoniously to Russia. Violence has returned to the streets with the occupation of government buildings in eastern and southern Ukraine.

Yesterday, there was a dramatic escalation in the crisis after firing broke out as pro-Russian separatists tried to storm local administration buildings in the city of Kramatorsk, in the Donetsk region.

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Last night Ukraine's Interior Minister, Arsen Avakov, said the government considered yesterday's attacks in the east of the country as an "act of aggression by Russia". "Units of the interior and defence ministries are implementing an operational response plan," he was quoted as saying.

Earlier yesterday armed men in camouflage seized the police headquarters in Slavyansk also in the Donetsk region, about 93 miles from the border with Russia.

The pro-Russian gunmen were reported to have seized at least 400 handguns and 20 automatic weapons from the police station. "The aim of the takeover was the guns," a police statement said. "They are giving these guns to participants in the protest in Slavyansk."

According to the new pro-Europe Ukrainian leadership in Kiev, the takeover of government buildings in eastern and southern areas is part of a plan drawn up by the Kremlin to dismember Ukraine.

Responding to the escalating crisis, Ukraine's interim prime minister, Arseniy Yatsenyuk, arrived in eastern regions on a mission to reassure business leaders and local politicians that "no-one, nowhere, no how will ever restrict the use of the Russian language" - a statement intended to calm fears among Russian speakers that Ukrainian nationalists in the new government want to outlaw the language.

Above all, Yatsenyuk wants to regain lost ground to head off the threat of the federalisation of Ukraine ahead of this week's crucial talks involving Russia, Ukraine, the EU and the US to settle the current crisis. The talks have been arranged to try to resolve tensions created by the collapse of the Ukrainian government in February and Moscow's move to impose its influence on the Russian-speaking areas of this vital geo-strategic region.

Although Russian and US diplomats have been cautious about the outcome of the conference, and warn that a breakthrough is unlikely, Russia's president Vladimir Putin was in bullish mood when he welcomed the fact that a conference has been arranged at all, with Vienna or Geneva being the most likely location.

"I hope that the initiative of the Russian foreign ministry on adjusting the situation and changing it for the better will have consequences, and that the outcome will be positive," Putin told a televised government meeting last week. "At the very least, I hope that the acting [leaders] will not do anything that cannot be fixed later."

Putin was right to display confidence, holding most of the aces in a game already stymied by both sides last week accusing the other of stoking tensions in the region. Broadly speaking, Russia is attempting to face down Ukraine, which has the backing of the EU and the US. There is nothing new in this - diplomacy has always been as much a game as an armed struggle - but the stakes on this occasion are worryingly high. Both sides have taken steps to strengthen their position and neither shows any sign of taking a backward step.

Having taken possession of Crimea through a largely peaceful referendum that only just managed to stay on the right side of international law, Russia has flexed its military and economic muscles to show the rest of the world that it has no intention of backing down. The first move came last Thursday when new satellite images released by Nato appeared to show an estimated 40,000 Russian troops assembling on the border with Ukraine, apparently on a war footing. This was followed by a war of contradictory words.

At first Moscow claimed they were only taking part in a pre-arranged exercise and going about their usual business but that did not impress Nato Secretary-General Anders Fogh Rasmussen who criticised Moscow for "stirring up ethnic tensions in eastern Ukraine and provoking unrest".

Then the story changed with the Russians claiming the photos were not taken recently but were, in fact, historical images from an earlier period. "The photos distributed by Nato show units of the Russian Southern military district, which were conducting various exercises last summer, including near the Ukrainian border", said the Russian military's General Staff in a statement.

The revelation seemed to back up an earlier assertion made by US Secretary of State John Kerry following talks with Russian foreign minister Sergei Lavrov that the Russians were ready to "de-escalate" along the border - but this was not the end of the story.

In an attempt to win back lost ground on Friday night Nato issued a new set of images they claimed were recent and showed evidence of a temporary base for ground forces, strike aircraft and helicopters at Novocherkassk and Kuzminka. By comparing various sets of "before and after" images and their dates a Nato spokesperson was able to demonstrate that the build-up had, in fact, occurred in the past few days.

"This is a capable force, ready to go," said Brigadier Gary Deakin, a British officer who runs Nato's crisis operations and management centre near Mons in Belgium. "It has the resources to move quickly into Ukraine if it was ordered to do so. It is poised at the moment, and it could move very fast."

Deakin also insisted that the force had the capability to strike further afield and was certainly powerful enough to overcome Ukraine's larger army of 130,000 troops. Meanwhile, pro-Russian activists calling themselves the Army of the South-East are still defying a Ukrainian ultimatum to vacate state buildings occupied by them in the eastern cities of Donetsk, Kharkiv and Luhansk. Far from taking up the Ukrainian government's offer of unconditional amnesty, the protestors have asked Moscow to intervene to protect them.

This comes in response to a recent Russian assertion that Moscow has the right and the obligation to protect Russians anywhere in the world. Known in diplomatic circles as the "Putin Doctrine", this came to the fore last week when Dmitry Peskov, spokesman for Putin, announced on Russian TV that "Russia is the country on which the Russian world is based" and that Putin "is probably the main guarantor of the safety of the Russian world". A protest banner hanging from one of the occupied buildings in Donetsk read: "Russia, save us from slavery!"

More worryingly, Putin has taken the opportunity to exert further pressure on the west by reminding European leaders that he controls the taps to their gas supplies and can turn them off at will. The threat was made over unpaid Ukrainian fuel bills thought to be as high as US$2.2 billion, a move that could have a knock-on effect for at least 18 European countries. This prompted an accusation from the US State Department that the Russian leader was using fuel as a "tool of coercion" and that there could be a response through the use of increased economic sanctions.

The combination of military and economic muscle is being assembled for one purpose: to impose the Putin Doctrine in an area where it can yield results.

Since taking over control of Crimea, Moscow has made no secret of its ambitions to protect similar Russian-speaking communities in southern and eastern Ukraine, which are believed to number five million people. Russian nationalists refer to this region by the historical name "Novorossiya" or "New Russia" and claim that it was added to the Russian Empire by military conquest from the Ottoman Empire in the 18th and 19th centuries.

Today it is still considered to be vital to Russia's strategic needs. Speaking at the opening spring meeting of the Parliamentary Assembly of the Council of Europe in Strasbourg, Alexei Pushkov, the head of the State Duma committee on international affairs, said Novorossiya is still an important part of the Russian homeland and that it must be protected.

He told delegates: "The situation in eastern Ukraine shows that stability will not be achieved in Ukraine without heeding the wishes of the people who live in the eastern and southern regions of Ukraine, primarily, Russian-speaking people. It's a real problem and the people are really seriously unhappy there. They came to the conclusion that the Kiev administration does not fully control the situation, and the situation when there are military units that do not report to anyone and are illegal is not characteristic of a country that claims to be democratic."

Also in Moscow's sights is the neighbouring Moldovan region of Transdniester, which is home to at least 150,000 Russian speakers holding Russian passports. The region was brought in to the Russian Empire at the end of the 18th century and, following the October Revolution in 1917 and the creation of the Soviet Union, it became part of the Moldavian Soviet Socialist Republic. After the collapse of the Soviet Union, Transdniester declared independence in 1990 and two years later fought a secessionist war that was ended by ceasefire. Following Russia's recent annexation of the Crimea, government officials in Transdniester made a formal request to join the Russian Federation.

The next crucial date in the calendar will be the Ukrainian presidential elections due to be held on May 25 and the result could be decided by what happens this week in Vienna or Geneva. All that is known at present is that there will be two main candidates in former prime minister Yulia Tymoshenko and Petro Poroshenko, a successful businessman who has cut a deal with former heavyweight boxer Vitali Klitschko which allows him to stand.

Whoever wins the election will have to introduce reforms to rid the country of the corruption that led to the downfall of former president Viktor Yanukovych and provoked rioting in the capital Kiev.

They will also have to heal the breaches created by the demands of the pro-Russian factions in eastern and southern Ukraine. However, as last week's events have demonstrated, the matter might already have been taken out of their hands and the result might be decided by what Moscow wants for Ukraine. As Tymoshenko opened her campaign last week that possibility certainly seemed her main fear.

"Ukraine has no single trump card, no real argument to stop military aggression," she said. "The only talks between Kiev and Moscow would be about complete capitulation."