Russia says United States Resorting to "Overt Blackmail"

· Ties will be hurt if measures hit its firms, it says

· Warns sanctions undermine efforts to rein in nuclear work

Russia sharply criticised 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 programme, called the measures "overt blackmail" and a "crude contradiction of international law."

The measures approved by Congress on Aug. 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.

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.