The United States and the International Monetary Fund offered Sunday to assist Ukraine in rebuilding its battered economy following devastating protests that have plunged the country into its worst crisis since independence.

Fears that Ukraine's debt-laden economy is facing default have sparked panic on markets, with bond yields rising sharply and the hryvnia currency losing a tenth of its value in the span of a few weeks.

US Treasury Secretary Jacob Lew said he held talks with Russian Finance Minister Anton Siluanov at the G20 meeting in Sydney after the Ukraine parliament voted Saturday to oust President Viktor Yanukovych, ending a week of brutal violence in the capital Kiev.

"The United States, working with other countries, stands ready to assist Ukraine with its efforts to return to a path of democracy, stability and growth," Lew told a press conference.

"We hope that the resolution of the violence in Ukraine will lead to a new inclusive multiparty, technocratic government willing to make the economic reforms needed.

"The IMF remains in the best position to help states like Ukraine deal with their economic challenges," Lew said.

IMF chief Christine Lagarde said after the G20 meeting that the IMF would also stand ready to help Ukraine, noting the country is an IMF member.

"So if the Ukrainian authorities were to ask for IMF support, whether it's policy advice, whether it's financial support together with economic reform discussions, obviously we stand ready to do that.

"We will be ready to engage, ready to help with the diagnosis of the current situation... before we of course try to go further and play the catalytic role that the IMF typically plays in such situations," Lagarde said.

Russia aid in doubt

There is uncertainty over whether Moscow will keep paying out a $15-billion bailout it promised to Kiev, with the latest tranche of Russian aid suspended pending a return to calm.

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

In Washington, a senior US State Department official said Secretary of State John Kerry had spoken with Russian Foreign Minister Sergei Lavrov and agreed the need "to resolve the situation on the ground without violence".

The Secretary of State "expressed the importance of encouraging Ukraine to move forward on a path towards constitutional change, de-escalation, the creation of a coalition government, early elections and rejection of violence", the official said.

Kerry "reiterated the view that Russia, the United States and appropriate European and international organisations must work together to support a strong, economically prosperous, unified, and democratic Ukraine", the official said.

US President Barack Obama held "constructive" talks Friday with Russian counterpart Vladimir Putin as he pressed for swift implementation of a deal to end Ukraine's deadly crackdown on protesters.

Putin and Obama agreed that the Ukraine agreement needed to be swiftly implemented and that all sides should refrain from violence, as they also pressed for the need to stabilise the economy.

Washington had offered staunch support for protesters in Kiev, demanding political concessions from the Moscow-backed Ukraine government and warned of "consequences" if violence, which has killed about 100 people, did not stop.