The Group of Seven most developed economies on Wednesday said a Moscow-backed referendum in Crimea on switching over to Kremlin rule would have "no legal effect" and called on Russia to back down.

The G7 -- Britain, Canada, France, Germany, Italy, Japan and the United States -- along with European Union leaders 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," they 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 recognize the outcome."

The statement comes ahead of Sunday's referendum in the strategic Black Sea peninsula and as US President Barack Obama is set to welcome Ukrainian Prime Minister Arseniy Yatsenyuk to the White House on Wednesday.

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

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

The G7 and the EU said annexation of Crimea would be a "clear violation" of the United Nations 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," they warned.

They called on Russia to de-escalate the crisis by moving forces back to bases in Ukraine and reducing numbers to pre-crisis levels, opening talks with Kiev and using international mediators to address concerns.

"We also remind the Russian Federation of our decision to suspend participation in any activities related to preparation of a G-8 Sochi meeting until it changes course and the environment comes back to where the G-8 is able to have a meaningful discussion," they said. (AFP)