US says Crimea referendum will have 'no legal effect'

President Barack Obama has said the US will stand by Ukraine in an effort to help it maintain territorial integrity and sovereignty.

After meeting with Arseniy Yatsenyuk, Ukraine's prime minister, at the White House on Wednesday, Obama said the West will be forced to apply a "cost" to Moscow if it does not change its course in Crimea.

Yatsenyuk said his country "will never surrender" to Russia and maintained his stance that Ukraine is ready for talks with Moscow.

He said that Ukraine "is and will be part of the Western world".

The Crimean regional government has scheduled a referendum for Sunday on whether to separate from Ukraine and join the Russian federation.

But Obama said that the US "will not recognise any referendum that goes forward" and expressed hope that last-minute diplomatic efforts might lead to a "rethinking" of Sunday's vote.

The annexation of Crimea by Russia would be a violation of the UN Charter and the referendum on the subject had no legal basis, the G7 group of nations also told Russia.

On Wednesday, the G7 called on Russia "to cease all efforts to change the status of Crimea contrary to Ukrainian law and in violation of international law", according to a statement released by the White House.

Any such referendum would have no legal effect," the group said. "Given the lack of adequate preparation and the intimidating presence of Russian troops, it would also be a deeply flawed process which would have no moral force. For all these reasons, we would not recognise the outcome."

The G7, the UK, Canada, France, Germany, Italy, Japan and the US - along with EU leaders, said annexation of Crimea would be a "clear violation" of the UN Charter, and would violate Russia's commitments under several other treaties.

"Should the Russian Federation take such a step, we will take further action, individually and collectively," the group said.

Russian troops moved into Crimea, a predominantly ethnic Russian region, after the February 22 removal in Kiev of Ukraine's pro-Kremlin leader Viktor Yanukovich.

The move by Moscow has led to the biggest breakdown in relations with the West since the Cold War.