It was a lurking fear that was yet to take its full grip.

A few weeks ago in Donetsk, Ukraine's fifth-largest city, I witnessed pro-Russian separatists making their presence felt, though the real fighting was still contained to surrounding eastern Ukrainian towns like Sloviansk and Kramatorsk.

On Donetsk's tree-lined Pushkin Boulevard, only yards from the city hall occupied by separatist activists, young fashionable Ukrainians still hung out in the cafes drinking locally brewed beer and eating Borscht.

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This weekend, however, some of those same streets are barricaded, cafes closed, and their customers have fled the city fearing that all-out war between the Ukrainian army and separatist fighters is just around the corner.

"We are leaving for Lviv by train and it's full of other people getting out of Donetsk," read the text message on Friday from the man who had been my fixer and translator in eastern Ukraine.

His decision to leave, along with his partner and family for the town of Lviv in the far west of the country near the Polish border, was prompted purely by the fear that Donetsk is about to be engulfed by fighting. His concerns were born out by another journalist colleague, who works for the broadcast channel France 24, who has also just left Donetsk. "It's about to get ugly," was how he summed it up.

For many locals the situation had already turned ugly, brought home by last week's fiercely fought battle for the city's airport when dozens of separatists died.

Where last month I encountered the scruffy masked men wielding crowbars and clubs that guarded the city hall headquarters of the separatists' Donetsk People's Republic, others have since arrived in armoured personnel carriers carrying much more lethal weapons. Something is clearly shifting in Donetsk. No sooner last week had separatists around Sloviansk shot down a Ukrainian military helicopter, killing 12 including a general, than the same day the shadowy Vostok Battalion is said to have set up base in the 11-storey Donetsk City Hall building.

While reports of the Vostok Battalion's ominous presence is difficult to fully confirm, several videos appear to have documented its presence. One video, uploaded to Russian social media site VKontakte, showed an armoured personnel carrier driving through Donetsk, carrying a Russian flag and a Vostok Battalion inscription that also appeared elsewhere in the city.

Ramzan Kadyrov, the Kremlin-backed leader of Chechnya, has denied sending Vostok fighters, but said some could have gone of their own accord. "We did not send them. They are absolutely not our staff," Kadyrov said in an interview shown on Russian television show, Vesti, yesterday.

"If a person voluntarily leaves (Russia), we have no right to stop him. This is his own decision."

But Kadyrov did not rule out sending Chechen fighters to Ukraine if ordered to do so by Russian president Vladimir Putin, who reserves the right to use force in Ukraine if its Russian-speaking minority is in danger.

"If there is an order, we will fulfil it with pleasure, because any warrior is a defender of his people, his Fatherland," Kadyrov said.

"If we imagine there's about 14 Chechens in Donetsk and they have already caused such a stir, what happens if a battalion is sent there?"

What happens indeed, for within Kadyrov's question there also lies an inherent warning. Whatever the official denials from Kadyrov or the Kremlin there is little doubt that Chechens have been coming to eastern Ukraine to help separatists in their fight against the Kiev authorities. They are not the only ones. That much I established from interviews conducted in Sloviansk last month where I met pro-Russian activists who had volunteered to return and join the fight.

Even if Chechen fighters are not in and around Donetsk in any numbers, the very rumour of their presence is sure to create fear given their notorious reputation for brutality. Kadyrov's security services have long been accused of human rights abuses such as kidnappings and torture in mainly Muslim Chechnya, though his aides deny it.

Yesterday, there was more fighting around Donetsk airport where according to the self-styled Donetsk People's Republic separatist leader, Denis Pushilin, six rebels were killed while trying to collect the bodies of comrades who had died under Ukrainian army fire.

The Kiev authorities' efforts to retake Donetsk airport was the first time the Ukrainian side had unleashed its full force on the pro-Russian two-month rebellion, caused partly by the ousting of a Moscow-friendly president and the annexation of the Crimean peninsula by Russian troops.

"They died trying to take back the Ukrainian airport, and our boys were trying to get their bodies out," Pushilin said over the telephone, without giving further details of the violence. He said the six had died during the day on Friday.

Though fighting has eased in Donetsk, the standoff over the city airport remained unresolved yesterday with amateur video footage showing a cargo plane taking off and releasing decoy flares as it was fired at from the ground.

Regional news agency Interfax cited Alexander Boroday, the self-styled prime minister of the republic, as saying the separatists will ask the International Red Cross to remove corpses by the airport. It was unclear how many remained.

A spokesman for what the Kiev government has called an "anti-terrorist operation" said earlier yesterday two new attacks on the airport had been repelled by Ukrainian forces with no injuries to their side. It was not clear whether the attacks were in fact attempts by separatists to clear bodies.

In Donetsk itself, 2000 separatist supporters gathered in the city's central Lenin Square yesterday, waving Russian flags and chanting "Russia, Russia," in support of the self-proclaimed republic.

Further away from the centre, rebels reinforced two new barricades by the sprawling residence of multi-billionaire Rinat Akhmetov, Ukraine's richest man, and by the city hall where the Vostok Battalion appears to have set up their headquarters. All of this does not bode well for Donetsk and its citizens, where fear has taken hold. As my former translator warned in his text message: "It feels like the calm before the storm."