Europe and the United States on Monday mulled a financial aid plan as part of a political solution to Ukraine's crisis while President Viktor Yanukovych returned to work after four days of sick leave.

EU foreign policy chief Catherine Ashton, who is expected in Kiev this week, told the Wall Street Journal that the exact amount had not not been decided yet but said "the figures won't be small".

She said the plan, which could be discussed at a meeting of EU foreign ministers on next Monday, would look at "what we need to do in different parts of the economy right now to make things better."

Opposition leader Arseniy Yatsenyuk said the minimum required was $15 billion (11 billion euros) -- the same amount promised by Russia in a bailout that is now on hold during the standoff, with only $3 billion released to prop up Ukraine's economy so far.

Yatsenyuk has said Ukraine needs a 'Marshall Plan', referring to the Cold-War initiative which saw the United States help rebuild Europe after World War II to prevent the spread of Communism.

Two months of deadly protests have hit an already fragile economy that had been emerging from recession and sparked fears of a currency collapse as Ukrainians rush to buy up euros and dollars.

The protests have also created a diplomatic rift between Brussels and Washington on one side and Moscow on the other, with Russia accusing the West of meddling and supporting far-right extremists.

The Russian foreign ministry on Monday said Ukraine's protesters should "renounce threats and ultimatums" after one opposition lawmaker called for the formation of "self-defence units".

Yatsenyuk, a former economy minister, said Russia had created "a mechanism that prevents Ukraine from integrating with the EU" by making it dependent on cheap gas supplies from the east.

The 39-year-old politician also said the possible Western aid would be linked to a broader deal including constitutional reforms to take away Yanukovych's powers.

"Right now everything is monopolised here," he said.

Boosting parliament's powers is one of the key demands of the opposition from Yanukovych, who returned to work on Monday after granting some concessions but failing to ease the crisis.

The opposition has so far refused to lead a new government despite being invited to do so and there are no meetings currently planned with Yanukovych.

A spokesman confirmed the president was back at work on Monday after suffering from "an acute respiratory infection" -- seen by critics as a ploy to win time.

The protest movement has called for "international mediation" in the negotiations.

Yanukovych has so far scrapped hugely controversial anti-protest laws and the prime minister and the entire cabinet have resigned under opposition pressure.

The list of unanswered opposition demands includes the immediate and unconditional release of detained activists and bringing forward a presidential elections scheduled for 2015 to this year.

Over the past few weeks, the protests have expanded beyond the capital and the traditionally pro-opposition western Ukraine into pro-Yanukovych, Russian-speaking central and eastern regions.

Ukraine's worst political crisis since its 1991 independence broke out after Yanukovych ditched a key pact with the European Union in November.

The protest movement has recently radicalised and turned into a drive to oust the 63-year-old leader.

The opposition claims police abuses are widespread.

A prominent Ukrainian protester, whose account of being kidnapped, tortured and dumped in a forest outside Kiev shocked Europe, left Ukraine for treatment in Lithuania on Sunday.

Dmytro Bulatov, 35, said he was "crucified" by his unidentified captors and television images filmed shortly after his release showed his badly swollen face caked in dried blood.

He was only allowed to leave with last-minute authorisation from a Kiev court as he still faces criminal charges for organising "mass unrest".

Ukrainian authorities have cast doubt on the veracity of his story and Foreign Minister Leonid Kozhara dismissed his injuries as "a scratch" before retracting the comment.

The case has been highlighted by the opposition as an example of what it says is a "secret repression" against protesters in which pro-government vigilantes have been employed.

According to official figures, four people -- two protesters and two policemen -- were killed at the height of the clashes last month that turned parts of central Kiev into a veritable war zone. (AFP)