THE death of a Ukrainian soldier in an attack on a base in Crimea, just hours after President Vladimir Putin signed a treaty recognising the region as Russian, has been denounced as a war crime.

The denunciation was made by Arseniy Yatsenyuk, Ukraine's pro-Western Prime Minister, who compared the attack by masked men and the annexation of Crimea to Nazi Germany's takeover of Austria and Sudetenland in the run-up to the Second World War.

Mr Yatsenyuk said he had ordered Ukraine's defence minister to call a meeting with his counterparts from Britain, France, and Russia, signatories to a 1994 treaty guaranteeing Ukraine's borders, to "prevent an escalation of the conflict".

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The death in the Crimean capital of Simferopol was the first casualty on the peninsula to be caused by a military clash since it came under Russian control three weeks ago. A second man was injured.

A Ukrainian spokesman said the attackers had told the Ukrainian soldiers they were under arrest and confiscated their documents. He said those behind the killing were "unknown forces, fully equipped and had their faces covered".

Unconfirmed reports last night from the Crimean news agency, Kry­minform, said a pro-Russia defence force member had been shot dead later on.

William Hague, the Foreign Secretary, said the attack "underlines the grave dangers in this tense situation". Earlier, David Cameron denounced Russia's annexation of Crimea, saying: "It is unacceptable for Russia to use force to change borders, on the basis of a sham referendum held at the barrel of a Russian gun."

Britain has suspended military co-operation and arms exports to Russia amid fears of a military escalation in the Ukraine.

No 10 made clear the UK Government would be pressing for further measures to be taken against Russia when EU leaders meet tomorrow in Brussels.

Mr Cameron's spokesman said preparatory work on additional sanctions was under way, but declined to elaborate. He stressed the Government supported the call by President Barack Obama that a G7 meeting of leaders from the US, the UK, Germany, France, Italy, Japan and Canada be held on the back of a nuclear security summit due to be convened in The Hague next week.

The EU has set up a three-phase approach under which sanctions would be ratcheted up if Russia failed to rein itself in. On Monday, the second phase was activated with the announcement of travel and asset restrictions on 21 key figures in the Russian and Crimean regimes. Britain believes it is now time to move to the third stage, which includes wider economic sanctions against Russia. The US has also imposed financial restrictions and warned of more to come.

In Moscow, Mr Putin and the leaders of Crimea enraged the West when they signed a Bill that absorbed the region into Russia following the results of Sunday's disputed referendum, in which 97% of voters backed secession.

The Russian President accused the West of behaving irresponsibly in backing an "extremist" uprising to oust pro-Moscow President Viktor Yanukovych.

"In the hearts and minds of people, Crimea has always been, and remains, an inseparable part of Russia," he declared.

The Russian Foreign Ministry insisted the people of Crimea had "made their democratic choice in line with the international law and the UN Charter, which Russia accepts and respects".

Nato Secretary-General Anders Fogh Rasmussen said: "Russia has disregarded all calls to step back into line with international law and continues down the dangerous path. He said Nato allies would not recognise Crimea's "illegal and illegitimate" annexation.