Western powers and Russia squared off over crisis-hit Ukraine as a top EU envoy prepared to fly to Kiev on Monday to buttress its new leaders' tilt away from Moscow.

Russia's fury at the breakneck pace of change in the ex-Soviet nation that saw president Viktor Yanukovych ousted following a week of carnage was on full display with Moscow's recall of its Kiev ambassador and freeze of its $15-billion Ukrainian bailout loan.

Both the United States and Britain warned Russian President Vladimir Putin not to even consider using force to regain sway over a neighbour he views as vital to his efforts to build an economic -- and possibly even military -- counterweight to the West.

Resurgent opposition leaders in parliament meanwhile prepared on Monday to cobble together a new cabinet after dismantling the last vestiges of a Yanukovych regime that many in the nation of 46 million viewed as both inept and corrupt.

The country's new prosectors opened criminal probes into top security officials -- most of them in hiding since Saturday -- they held responsible for ordering police to use snipers against protesters in central Kiev in carnage that killed nearly 100 and left the nation in profound shock.

The heart of Kiev was a site of deep mourning late on Sunday as thousands streamed in clutching candles and flowers to pay their respects to those who fell in defence of Ukraine's aspirations to be rejoined with the West.

Some people shook their heads in disbelief while inspecting lamp posts studded with bullet holes. Others joined priest in requiems as songs of mourning rose over the ancient city.

Russia's vocal displeasure at the changes hitting its neighbour has translated into Western anxiety about Putin actually ordering tanks into Ukraine in an echo of what the Soviets did in former Czechoslovakia and Hungary during the Cold War.

US National Security Adviser Susan Rice said bluntly that sending troops to restore a Kremlin-friendly leadership in Kiev "would be a grave mistake".

Britain's Foreign Secretary William Hague also stressed that "any external duress on Ukraine any more than we've seen in recent weeks... it really would not be in the interests of Russia to do any such thing."

But Hague, when asked about the possibility of Putin ordering tanks into Ukraine, said: "We don't know, of course, what Russia's next reaction will be."

Russia's anxiety was underscored late on Sunday when the foreign ministry recalled ambassador Mikhail Zurabov "due to the escalation of the situation in Ukraine".

The foreign ministry also said Foreign Minister Sergei Lavrov had flatly told US Secretary of State John Kerry that Russia strongly condemned "the seizure of power" by the opposition in Kiev.

Yet the diplomatic sparring was likely to only intensify on Monday with the arrival in Kiev of EU foreign policy chief Catherine Ashton -- a prime Russian target of charges that the West was meddling in Ukraine's internal affairs.

Ashton's office said the two-day visit would focus on finding "a lasting solution to the political crisis and measures to stabilise the economic situation."

Ukraine's new interim leader Oleksandr Turchynov had warned on Sunday that Kiev would have no choice but to default on $13-billion in foreign obligations due this year should the West fail to fill in for Moscow's suspended aid.

Turchynov on Sunday vowed to draw up a "government of the people" and warned Russia that he expected the Kremlin to respect his country's pivot to the West.

"We are ready for a dialogue with Russia... that recognises and takes into account Ukraine's European choice," the 49-year-old said in a television address.

He has until Tuesday to piece together a coalition cabinet and come up with a prime minister willing to take up the challenge of keeping Ukraine from falling off the economic cliff before new presidential polls are held on May 25.

Gone are the old interior minister and prosecutor general -- both hate figures in the opposition for their roles in ordering police to open fire on protesters -- as well as a foreign minister who had guided Ukraine's path back toward Russia.

Turchynov is a close ally of Yulia Tymoshenko -- an iconic but divisive former premier whom parliament on Saturday released from a controversial jail sentence she was handed by Yanukovych's team -- and is not himself expected to challenge for the presidency.

Tymoshenko began to build up her credentials on Sunday by fielding a telephone call from German Chancellor Angela Merkel and meeting a string of Western ambassadors in Kiev.

But her spokeswoman stressed that the 53-year-old -- who had appeared before the crowd in a wheelchair on Saturday because of back problems -- had made no decision about running in May.

"This is not the right time for this," spokeswoman Natalia Lysova told AFP.

The opposition's main presidential challenge had been mostly expected to come from boxer-turned-lawmaker Vitali Klitschko.

The popular UDAR (Punch) party leader had initially announced his presidential ambitions in October. But he backtracked from those comments on Sunday in an apparent concession to Tymoshenko's continued public appeal.

"My main goal is not to take the chair of president," Klitschko told the BBC.