Pro-Russian separatists seized six armoured personnel carriers from the Ukrainian army on Wednesday, which they then drove in a victory lap through the centre of two towns held by militia units.

About 100 heavily armed men, some in balaclavas and wearing military fatigues, rode on top of the seized armoured vehicles, the first of which was flying a Russian tricolour. Several hundred locals gathered around the convoy cheering, tooting their car horns and waving in support as it rolled past Kramatorsk's railway station, not far from the airfield where Ukrainian soldiers clashed with separatists on Tuesday.

Ukrainian military helicopters hovered above the dramatic scenes but there seemed to be no attempt by government forces to try to wrest back control of the situation. The seized armoured personnel carriers were driven to Slavyansk, where a Russian flag had been raised above a checkpoint at the city entrance. A plane resembling a Su-27 circled low over the town's square.

It came as Nato announced it would step up its presence near Russia's borders to reassure its eastern European member states, with more air patrols over the Baltic states and warships in the Baltic Sea and eastern Mediterranean.

The Ukrainian troops and armour had arrived in Kramatorsk earlier on Wednesday, where pro-Russian militia seized the vehicles with no shots fired. The Ukrainian defence ministry confirmed that vehicles had been taken by pro-Russian forces.

The pro-Russian militiamen who drove the troop carriers into Slavyansk refused to say where they had got them. "From space," one said. "They came on their own," said another.

One soldier siding with the separatists in Slavyansk told a Reuters reporter that he and others in his group were part of a Ukrainian paratroop unit who could not shoot "our own people".

Pro-Russian protesters seeking independence from Kiev have occupied at least nine government buildings in the region for more than a week – but this is the first time that separatist forces deep inside Ukraine have managed to seize heavy military equipment, and a further sign that the situation in the east is slipping out of Kiev's grip.

Locals gathered as the militiamen parked the vehicles near city hall. A pair of women recognised one man and hugged him, suggesting that at least some of them were local.

The new "people's mayor", Vyacheslav Ponomaryov, arrived and greeted the men, then led a group of them off the square towards other occupied buildings.

But not all the locals who had gathered joined the hero's welcome. One man who identified himself only as Valery angrily asked the militiamen, who were enforcing a wide perimeter around the armoured vehicles, what they were doing.

"Part of the population supports them," he said. "But people who work, like me – I'm an entrepreneur – they don't want this."

Valery said he did not support calls for a referendum and wanted to vote in the presidential elections planned for 25 May, which many here say they will boycott.

"People think everything in Russia is spread with honey," Valery said – a statement that provoked angry exclamations and arguments from nearby crowds.

Later about 40 to 50 Ukrainian paratroopers who had been on been on the vehicles were released from Slavyansk city hall and loaded onto two buses. They said they were heading to the neighbouring region of Dnipropetrovsk, where their 25th regiment is based.

The troops carried rucksacks and many of them kept their weapons, but they looked defeated. "What were we supposed to do? Shoot peaceful protestors?" one soldier told the Guardian when asked why they had chosen to leave.

He said the soldiers were properly equipped and supplied, denying that they were going hungry.

Crowds look on as pro-Russian separatists drive round the city on armoured vehicles. Photograph: Luke Harding

Some of the Ukrainian troops stayed to join the pro-Russian militia, the soldier said. This was confirmed by a rebel commander, who declined to say how many had stayed.

Also on Wednesday, another column of Ukrainian armour was stopped in its tracks in a village outside Kramatorsk by a crowd of locals who bought the men bread and sausages. Episodes of Ukrainian troops being stopped by locals have played out several times in recent days.

Ukrainian government forces launched their first significant military action in the east of the country on Tuesday, clashing with about 30 pro-Russian gunmen at a provincial airfield and heightening fears that the standoff could escalate into a major armed conflict.

Ukraine's acting president said the recapture of the airport was just the first such action aimed at restoring Kiev's control over the east.

Russia's president, Vladimir Putin, declared the Ukrainian moves "anti-constitutional acts" and in a phone call to the United Nations secretary general, Ban Ki-moon, demanded that the UN condemn them. But the US voiced strong support for the Ukrainian operation, arguing that the government in Kiev had to respond to armed groups.

Representatives from Ukraine, Russia, the US and the European Union are due to meet in Geneva on Thursday for the first time since the crisis began in February.