Russia sharply criticised new US sanctions against Iran yesterday, 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 UN Security Council to pressure Tehran over its nuclear programme, called the measures “overt blackmail” and a “crude contradiction of international law”.

The US ceased most trade with Iran many years ago and has put increasing pressure on 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 US 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.

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 programme - Russia, the US, China, Britain, France and Germany - and hurts the chances of success.