Moscow delivered a damning indictment of post-revolutionary Ukraine on Monday, denouncing alleged discrimination of the ethnic Russian minority, accusing the west of sponsoring a takeover of the country by "terrorists" and "extremists", and clashing with Washington over plans for early elections in May.

"Russia is extremely concerned about the situation in Ukraine," said a foreign ministry statement, which followed the highest-level reaction from Moscow so far to the collapse of Viktor Yanukovych's presidency. Dmitry Medvedev, the prime minister and former president, accused the post-Yanukovych authorities and parliament of lacking legitimacy.

"If you consider Kalashnikov-toting people in black masks who are roaming Kiev to be the government, then it will be hard for us to work with that government," Medvedev said. "Some of our foreign, western partners think otherwise, considering them to be legitimate authorities. I do not know which constitution, which laws they were reading, but it seems to me it is an aberration … Something that is essentially the result of a mutiny is called legitimate."

The blistering language from Moscow, the first real response to the crisis from Ukraine's neighbour and former overlord, collided with western attempts to keep the Russians onside for the looming challenge of establishing a stable, inclusive government and coming up with a multibillion-dollar rescue to try to stabilise an economy in freefall.

The new interim leaders of Ukraine said the country needed $35bn (£21bn) over the next two years and fast action on a bailout, as Moscow signalled its was freezing its $15bn in cheap loans – the package offered in November when Yanukovych triggered the uprising by turning to Russia and away from the European Union.

A wanted poster for Victor Yanukovich in Kiev Photograph: Yannis Behrakis/Reuters

The Russian foreign ministry statement pressed all the buttons that will have the west and Kiev alarmed about ethnic and religious strife fracturing the country in two. It complained that ethnic Russian rights were already being violated after the parliament rescinded the status of Russian as a second language.

"Referring merely to revolutionary expediency, [the parliament] is imposing decisions and laws aimed at repressing the human rights of Russian and other national minorities. There are even calls for a complete ban of the Russian language," it said.

Moscow opposed plans for early elections in May, calling first for constitutional changes and a referendum. This contrasted with the stance of the US state department, which announced that William Burns, the senior diplomat credited with engineering a recent breakthrough in US relations with Iran, would travel to Kiev. It said Burns would "urge the new government to take all steps necessary for free and fair presidential elections in May. He will urge the rapid formation of a national unity government."

The collapse of the Yanukovych administration late on Friday is a setback to Vladimir Putin in Moscow, while Europe and the west are seen as the international beneficiaries. But with the Ukrainian economy on the brink of insolvency, the west is now under pressure to deliver tens of billions in aid.

It was clear that the west would insist on a programme under the International Monetary Fund, which would come with very tight strings attached. But it appeared that the EU, US and other international actors would go beyond an IMF package with bilateral financial support. Berlin spoke of organising an international donors' conference to raise funds from Japan and China, while stressing that it wanted Russia as part of the funding team.

Dmitry Medvedev, the Russian prime minister. Photograph: Astakhov Dmitry/Itar-Tass/Corbis

The Obama administration has indicated it regards Ukraine as being under new management. Jay Carney, the main White House spokesman, said that although Yanukovych was elected his actions had cost him his legitimacy and he was "not actively leading the country at present".

The US said Burns would "be accompanied by representatives of the Department of the Treasury and the National Economic Council, who will work in concert with partners such as the EU and the IMF to discuss needed financial support while a new government implements the difficult steps necessary to reform the economy".

The US treasury secretary, Jacob Lew, told Arseniy Yatsenyuk, tipped as Ukraine's next prime minister, that the US, "together with Europe and others in the international community, are ready to supplement an IMF programme to cushion the impact of reforms," according to a treasury statement.

The British foreign secretary, William Hague, said he would visit Ukraine shortly, and said the country needed urgent international financial help. Hague is due to travel to Washington later on Monday to discuss Ukraine with the US secretary of state, John Kerry, and the IMF. He said the IMF was best placed to provide financial support and advice to Ukraine. "Ukraine's financial situation is very serious and without outside assistance may not be sustainable," Hague told parliament.

The Russians bitterly criticised everything that had happened in Ukraine since Friday, arguing that the events were at odds with the settlement terms agreed by Yanukovych and opposition leaders in 24 hours of tough negotiations mediated by three EU foreign ministers last Thursday and Friday.

The full Russian support for this agreement appeared odd since Moscow was also involved in the negotiations in the person of Vladimir Lukin, a former Russian ambassador to Washington, who left the talks hours early, and Russia was the only party present that refused to sign the settlement. Lukin abandoned the talks, describing the street protesters as "nationalists", "revolutionaries" and "terrorists".

The Russian statement on Monday said events in Ukraine since the weekend "discredit the initiators and their guarantors [EU], threaten civic peace, social stability and public safety. We are bound to note that several of our western partners show no concern for the fate of Ukraine, but for one-sided geopolitical calculations. There is no sign of real assessment of the criminal acts of extremists, including neo-Nazis and antisemites."