IZYUM, Ukraine  - Ukraine sent a large armoured column and special forces towards the flashpoint city of Slavyansk on Tuesday as Kiev’s fledgeling leaders tried to reassert control over the separatist east.

The seven buses, filled with 100 paratroopers, and 20 tanks and armoured personnel carriers represented the Western-backed team’s most forceful response to date to raids that have seen state buildings in nearly 10 cities across Ukraine’s rust belt fall under the control of pro-Russian militants.

They had moved within 40 kilometres (25 miles) of Slavyansk and set up a checkpoint that could control all traffic leading to Slavyansk - an economically depressed industrial city of 100,000 that has been under effective control of separatist gunmen since Saturday.

“They must be warned that if they do not lay down their arms, they will be destroyed,” Ukrainian Security Service (SBU) General Valeriy Krutov told a group of reporters tracking the sudden tank movements. The SBU anti-terrorism centre director alleged that the gunmen had received reinforced by a deployment of several hundred soldiers from the Russian army’s Main Intelligence Directorate (GRU).

Increasingly insistent Western charges of the Kremlin’s direct involvement in the spreading eastern unrest set a chilly tone to a “frank and direct” exchange on the crisis between US President Barack Obama and his Russian counterpart Vladimir Putin. US President Obama urged Russian leader Vladimir Putin in a phone call late Monday to press pro-Moscow groups to lay down their arms in Ukraine, as the Kiev government sought UN help to tackle the growing insurgency.

A White House statement said the telephone call came at Russia’s request and that Obama accused Moscow of supporting “armed pro-Russian separatists who threaten to undermine and destabilize the government of Ukraine”.

European foreign ministers meanwhile held back on unleashing punishing economic sanctions against Russia in hopes that EU-US mediated talks on Thursday in Geneva between Moscow and Kiev could help de-escalate the most dire East-West standoff since the Cold War.

While Ukraine’s interim leaders are under pressure from ex-Soviet master Russia, they are also starting to feel the heat from a Kiev public unhappy with their inability to mount an effective response.

Late Monday, several hundred nationalists set fire to tyres outside the parliament building on Monday evening demanding the interior minister’s resignation. Yet the untested leaders have no clear solution to the high-stakes challenge presented by the coordinated series of raids in the east, which began in the depressed industrial hubs of Donetsk and Lugansk and have since spread to nearby coal mining towns and villages.

Inaction in the face the insurgents’ aggression and tough talk by Moscow could potentially see the vast nation of 46 million break up along its historic Russian-Ukrainian cultural divide. But a forceful military response - its very feasibility in question due to poor morale and desertions riddling army ranks - could prompt a devastating counterstrike by Russian troops who are waiting to act on Putin’s vow to “protect” his compatriots in the neighbouring state.

Russian Prime Minister Dmitry Medvedev on Tuesday said Ukraine was on the verge of civil war as Kiev authorities launched a military operation against pro-Kremlin militants in the separatist east. “I will be brief: Ukraine is on the brink of civil war, it’s frightening,” the country’s former president was quoted as saying by Russian news agencies.

Ukraine’s acting President Oleksandr Turchynov told an agitated session of parliament, during which some erstwhile supporters questioned his leadership, that the country was facing an eastern enemy rather than domestic discontent.

“They want to set fire not only to the Donetsk region but to the entire south and east - from Kharkiv to the Odessa region,” the acting president said.

Kiev’s news programmes have spent much of the past day replaying footage from an attack by militants on a police station in the town of Gorlivka in which men in gas masks pelted the building with Molotov cocktails before smashing its windows with rocks and bats.

One clip showed a green-uniformed man who identified himself as a Russian colonel telling the local police force that it now must obey his orders and must wear orange-and-black ribbons symbolising Kremlin pride.

The Kremlin set nerves in Kiev further on edge on Monday by announcing that Putin had received “a lot” of requests from eastern Ukraine “to help, to intervene in some form”. That was seen as a an effort to create a pretext for an invasion.

The crisis was exacerbated further still by two episodes with a dash of Cold War-style intrigue: a confrontation in the Black Sea in which a Russian warplane “buzzed” a US destroyer, and a weekend visit to Kiev by CIA chief John Brennan that was confirmed by the White House.

This charged atmosphere prompted Putin to place a surprise call to the US President Barack Obama that both sides later said covered plenty of ground but appeared to chart no new course.

The White House said Obama accused Moscow of supporting “armed pro-Russian separatists who threaten to undermine and destabilise the government of Ukraine”.

A Kremlin account of Putin’s call with Obama said the Russian leader blamed the unrest on “the unwillingness and inability by the Kiev authorities to take account of the interests of the Russian and Russian-speaking population”.

Moscow’s Interfax news agency had quoted an unidentified source as saying Brennan had recommended that Kiev use force against the pro-Russian militants.

But White House spokesman Jay Carney blasted that claim and depicted Brennan’s Kiev visit as routine.