KIEV (AFP) - Ukraine's new masters called on the West on Monday to urgently organise a financial rescue for their listing economy, saying they need US$35 billion (S$44 billion) to stave off default, while jilted Russia threatened economic punishment.

"Most government workers have quit their posts. The coffers are empty. We don't have enough money to pay our debts," new interim leader Oleksandr Turchynov said.

After limping along for three years thanks only to loans, the financial situation now was "extreme," he said.

Interim finance minister Yuriy Kolobov said the US$35 billion in foreign assistance was needed by the end of next year. He urged Western nations and the International Monetary Fund to convene a donor conference in the next two weeks.

IMF chief Christine Lagarde said on Sunday "we stand ready" to help Ukraine, a vow echoed by the United States, Britain, France and Germany.

Russia, which has suspended a US$15-billion loan pledged in December to the previous Ukrainian government swept aside in the weekend's tumult, has made it clear that Kiev can no longer count on its help.

Russian Prime Minister Dmitry Medvedev on Monday challenged the legitimacy of the new, interim authorities appointed by Ukraine's parliament, saying they resulted from "an armed mutiny".

If Ukraine goes ahead, as expected, with signing the partnership deal with the European Union - whose scrapping by ousted president Viktor Yanukovych triggered the protests leading to his downfall - Moscow will punish it by raising import duties on Ukrainian goods, Russian Economy Minister Alexei Ulyukayev said.

A EU trade pact "is not compatible with" the free-trade arrangement within the Moscow-dominated Commonwealth of Independent States, he said.

"It's already clear that Ukraine faces recession," he pointed out.

The ratings agency Standard & Poor's predicted on Friday that if Russia scrapped its $15 billion loan, Ukraine would default on the US$13 billion of debt it is due to repay this year.

It is unclear how much aid the EU, barely recovered from its own financial crisis, would be willing to extend, and how quickly.

European elections in May - in which nationalist parties will bemoan financial largesse outside their borders - are likely to preclude any quick steps.

The European Union is ready to assist Ukraine economically "provided there is a reform programme," a spokesman for the bloc's executive said on Monday.

European Commission spokesman Olivier Bailly also stressed that the long-awaited EU-Ukraine deal - while still on offer - cannot be signed until after the Ukraine's scheduled May elections.

"We must let a transition process go to its final point" of elections set for May 25 "and once we have a government we will be ready to discuss again".

Ukraine's economy, worth US$157 billion last year, was already struggling with 7.5 per cent unemployment and low export demand for its metal and engineering products before the unrest of the past three months, according to the World Bank.

The currency, the hrvynia, was savaged by the political uncertainty in recent weeks. Early Monday, it reached its lowest level in five years, at 9.36 to the dollar as the market caught up with the flurry of weekend events, before recovering slightly to 9.18 in later trading.

The yield on its dollar-denominated 10-year bonds - considered a benchmark - jumped to 9.826 per cent last Friday, reflecting increased risk, but eased off a bit to 8.695 per cent on Monday.

Gas, imported from Russia and often a vulnerability for Ukraine, should not be of concern immediately, however. The European Commission estimates the country has a comfortable 11.5 billion cubic metres of gas in storage.

Analysts said that, while the focus is on quick financial help to Ukraine, long overdue reforms are needed - especially if it takes steps towards the European Union.

"Ukraine has lacked an anchor for economic and institutional reforms since the collapse of Communism," macro-economic research firm Capital Economics said in a briefing note.

If it is able to make the changes, Ukraine's future would be better served by moving closer to the EU than to Russia, said Mr Holger Schmieding of Germany's Berenberg investment bank.

"Russia is a threat rather than a model," he said. "A mismanaged and fragile economy dependent on the price of (Russian) oil is no match for the attraction of a vast and prosperous partner."