Potential U.S.sanctions on Iran over its nuclear program will "deal a blow" to Russian-U.S. relations, a senior Russian official said Sunday, presuming a hard-line stance before the long-waited meeting of the heads of the two states.

Russian presidential aide Yury Ushakov told reporters that U.S. sanctions on Iran would "run against international law and affect third countries."

Moscow could not accept if Russian firms and banks become potential victims of such unilateral actions from the U.S., Ushakov warned.

Russian President Vladimir Putin and U.S. President Barack Obama are to meet on the sidelines of the upcoming Group of 20 summit Monday in the Mexican city of Los Cabos.

The two leaders had agreed by phone in early May that they would meet for one and a half hours during the summit, Ushakov said, which would be the first since Putin returned to the top seat.

Putin's absence from the Group of Eight summit last month in the United States and Obama's no-show decision at an Asia-Pacific Economic Cooperation forum meeting in Russia's Vladivostok in September augured a possible cool-down of the already soured Russia-U.S. relations amidst Putin's tough words concerning the U.S.-led missile defense system in Europe.

A new round of talks between Iran and the six major world powers -- Britain, China, France, Germany, Russia and the United States -- is due on Monday in Moscow, a month after the last round of "six plus one" talks was held in the Iraqi capital Baghdad.