Washington and its allies stepped up pressure on Russia in the run-up to Crimea's Sunday referendum over secession from Ukraine.

US President Barack Obama welcomed Ukraine's coup "prime minister" Arseniy Yatsenyuk to the White House while the Senate foreign relations committee drew up legislation authorising the president to impose economic penalties on selected Russian officials.

The G7 group of advanced capitalist countries issued a joint statement declaring that they would not recognise the results of the referendum.

"We call on the Russian Federation to immediately halt actions supporting a referendum on the territory of Crimea regarding its status, in direct violation of the constitution of Ukraine," the statement read.

"Any such referendum would have no legal effect."

Crimea's parliament made clear on Tuesday that if as expected the electorate votes to become part of Russia, the peninsula would declare itself independent and propose becoming a Russian state.

White House spokesman Jay Carney said that Mr Yatsenyuk's visit was meant to signal "that we strongly support Ukraine, the Ukrainian people and the legitimacy of the new Ukrainian government."

Senate foreign relations committee chairman Bob Menendez said: "Putin has miscalculated by playing a game of Russian roulette with the international community, but we refuse to blink and will never accept this violation of international law."

Mr Yatsenyuk arrived in Washington with outstretched hands, telling his hosts that his administration needed financial assistance and help to defend itself against Russia, which is "armed to the teeth."

President Obama has promised Ukraine $1 billion (£603 million) in loan guarantees, which is intended to supplement additional assistance from the International Monetary Fund.

The European Union has pledged €10.8bn (£9bn) in assistance to Ukraine, falling well short of the €25.2bn (£21.1bn) in international rescue loans that Kiev says it needs over the next two years.

While the politicians talked tough in Washington, cold war military alliance Nato deployed two Awacs surveillance planes from bases in Germany and Britain to fly over Poland and Romania to monitor the situation in Ukraine.

"Our flights will not leave Nato airspace. Regardless, we can observe, we can look a very long way," said Nato spokesman Lt Col Jay Janzen in Brussels.