Russia vowed on Monday to unveil its own solution to the Ukrainian crisis that would run counter to US efforts and would appear to leave room for Crimea to switch over to Kremlin rule.

The unexpected announcement came as Ukraine's new pro-European leaders raced to rally Western support in the face of the seizure by Kremlin-backed forces of the strategic Black Sea peninsula and plans to hold a Sunday referendum on switching Crimea's allegiance from Kiev to Moscow.

Russian President Vladimir Putin's threat to invade Ukraine after a wave of deadly protests toppled a pro-Kremlin regime last month has set off the most explosive crisis in East-West relations since the Cold War.

US President Barack Obama and his European allies are urging Russia to call its troops in Crimea back to their barracks and launch immediate negotiations with the new Ukrainian leadership, which Putin claims rose to power thanks to an "unconstitutional coup".

Russian Foreign Minister Sergei Lavrov told Putin in a televised meeting Monday that proposals he had received from US Secretary of State John Kerry on resolving the standoff "do not suit us very much" and were "framed as if there exists a conflict between Russia and Ukraine."

He said Russia had prepared a series of counter-proposals that would "take into account the interests of all Ukrainians".

Lavrov said Washington was basing its solution on a recognition of Kiev's new leaders while Russia still considered the ousted Viktor Yanukovych as the legitimate president of Ukraine.

But Lavrov gave no indication about when or where Russia's proposals would be made public.

- Putin backs Crimea referendum -

Putin added new urgency to the standoff on Sunday by telling German Chancellor Angela Merkel and British Prime Minister David Cameron that he fully backed the actions being taken by the self-declared rulers of Crimea -- in power since an end-of-February seizure of the government by pro-Kremlin gunmen.

The Kremlin said Putin stressed "the steps undertaken by the legitimate authorities of Crimea are based on the norms of international law" -- a comment strongly hinting that Moscow was ready to annex Crimea after handing the peninsula to Ukraine as a "gift" when it was a part of the Soviet empire in 1954.

Merkel -- whose cautious approach to imposing sanctions on Moscow has clashed with the more hawkish positions of eastern European nations and Washington -- bluntly responded that the Crimean referendum was "illegal."

US ambassador Geoffrey Pyatt told reporters in Kiev on Monday that Washington "is not prepared to recognise any result of the so-called referendum".

The public vote will ask the predominantly ethnic Russian population to choose between swearing allegiance to Moscow and declaring greater autonomy from Kiev while remaining a part of Ukraine.

Ukraine's interim Foreign Minister Andriy Deshchytsya said that if Crimea's leaders "want more rights and authority, then we are ready to do this."

The country's interim Defence Minister Igor Tenyukh meanwhile said the nation's army -- already on full combat alert -- had launched training exercises aimed at evaluating how the heavily outnumbered force could resist an offensive from its nuclear-armed neighbour.

The diplomatic wrangling continued as Ukrainian Prime Minister Arseniy Yatsenyuk prepared to fly to the United States to meet Obama and address the UN Security Council.

His first meeting with the US leader on Wednesday should add credibility to Yatsenyuk's untested government and provide Ukraine with a chance to iron out the details of crucial economic relief for its struggling economy.

The White House said Obama will discuss an economic support package that has already seen Washington pledge a quick infusion of more than $1 billion and the European Union promise to issue 11 billion euros ($15 billion) over two years.

Kiev says it needs about 25 billion euros ($35 billion) through 2015 to keep the country running after Russia froze a $15-billion bailout it promised Yanukovych as his reward for rejecting an EU trade deal in November that initially sparked the protests.

- Split between Russia and China -

The escalating crisis has seen Obama vow to impose travel bans and asset freezes on Russian officials held responsible for endangering the territorial integrity of Ukraine.

US officials have stressed that Putin himself is not on that list but have also warned that Washington could pull out of a G8 summit the Russian leader is hosting in Sochi in June.

The European Union for its part has halted visa talks and threatened to impose tough economic sanctions unless Putin quickly opens talks with Kiev.

The UN Security Council later on Monday will hold another meeting on Ukraine amid a rare but potentially significant split between allies China and Russia.

Beijing on Sunday affirmed its support for the "sovereignty and territorial integrity" of Ukraine -- a stance that appears to signal that its own concerns about separatist movements in China outweigh the importance of preserving a united front with Russia on foreign affairs.

UN Secretary General Ban Ki-moon on Monday warned against "provocative rhetoric and hasty actions" as he appealed for dialogue on Ukraine.

In a separate development, NATO announced it would deploy reconnaissance flights over Poland and Romania to monitor the crisis in Ukraine.

Crimea's separatist government said on Monday it had invited the Organisation for Security and Co-operation in Europe (OSCE) to monitor the referendum even while keeping the group's military observers from entering the region at gunpoint.

The European security body said it did not recognise the invitation as valid because it had not come from an OSCE members state but only an autonomous region of Ukraine.