UPDATE 6-Russia, U.S. sign civilian nuclear pact

(Adds U.S. State Department spokesman paragraphs 12-13)

By Guy Faulconbridge

MOSCOW, May 6 (Reuters) - Russia and the United States signed a pact on Tuesday allowing the world's two biggest atomic powers to boost their nuclear trade and work on new ways to prevent the proliferation of nuclear weapons.

The civilian deal will open up the booming U.S. nuclear market and Russia's vast uranium fields to firms from both countries by removing Cold War restrictions that prevented bilateral trade potentially worth billions of dollars.

U.S. ambassador to Russia, William Burns, signed the deal with the head of Russia's state nuclear corporation, Sergei Kiriyenko, on the last full day of Vladimir Putin's presidency.

"The United States and Russia were once nuclear rivals -- we are today nuclear partners," said Burns.

At the 2006 Group of Eight summit in St Petersburg, President George W. Bush and Putin ordered ministers to reach a deal but it has faced opposition from some U.S. congressmen because of Russia's nuclear cooperation with Iran.

A 123 agreement, so-called because it falls under section 123 of the U.S. Atomic Energy Act, is required before countries can cooperate on nuclear materials.

It is critical to the Global Nuclear Energy Partnership, or GNEP, which the United States and Russia have discussed for more than a year as a way to expand peaceful nuclear energy development and mitigate proliferation risks.