By Steve Gutterman
MOSCOW (Reuters) - Russia sharply criticized new U.S. sanctions against Iran on Monday, saying the measures to punish banks, insurance companies and shippers that help Iran sell its oil would harm Moscow's ties with Washington if Russian firms are affected.
Russia, which has long opposed sanctions beyond those approved by the U.N. Security Council to pressure Tehran over its nuclear program, called the measures "overt blackmail" and a "crude contradiction of international law."
The United States ceased most trade with Iran many years ago and has put increasing pressure other countries to reduce their business with the Islamic Republic.
The measures approved by Congress on August 1 build on oil trade sanctions signed into law by Obama in December that have prompted Japan, South Korea, India and others to slash purchases of Iranian oil.
"We are talking about restrictive measures not only against Iran but also affecting foreign companies and individuals working with it, including in the hydrocarbon extraction and transport, petrochemicals, finance and insurance industries," the Russian Foreign Ministry said in a statement.
"We consider efforts to ... impose internal American legislation on the entire world completely unacceptable," it said. "We reject methods of overt blackmail that the United States resorts to in relation to the companies and banks of other countries."
"Those in Washington should take into account that our bilateral relations will suffer seriously if Russian operators ... come under the effects of the American restrictions," the ministry said.
Relations between Moscow and Washington improved after President Barack Obama moved to "reset" ties early in his term, but they have been strained by disputes over Syria and President Vladimir Putin's accusation of U.S. meddling in Russian politics.
In the statement, Russia repeated its argument that unilateral sanctions - as opposed to those approved by the Security Council, where Moscow has veto power - are counterproductive.
The "constant increase of pressure on Tehran" undermines unity among the six nations leading diplomatic efforts to rein in Iran's nuclear program - Russia, the United States, China, Britain, France and Germany - and hurts the chances of success.
Russia balances its role in the diplomatic attempts to ensure Tehran does not develop nuclear weapons with aid to the civilian nuclear program in Iran, where it built a nuclear power plant that came on line this year.
Talks between Iran and the six powers in Moscow in June failed to end the standoff over Tehran's nuclear activities including the enrichment of uranium which Western nations fear is part of a bid for weapons capability.
(Editing by Robin Pomeroy)