MOSCOW (AP) - Russian and U.S. officials signed a key agreement on civilian nuclear power Tuesday that could give Washington access to Russian technology and potentially hand Moscow lucrative deals on storing spent fuel.

The deal, signed on the eve of Dmitry Medvedev's inauguration as president, signals a reversal in policy for the U.S. administration on cooperating with Russia on nuclear issues. Cooperation had cooled in recent years, mainly due to disagreements over how to handle Iran's perceived nuclear threat.

"The U.S. and Russia were once nuclear rivals," U.S. Ambassador William Burns said after a signing ceremony. "Today, we are nuclear partners with unique capabilities and unique responsibilities for global nuclear leadership."

But the agreement ran into immediate trouble on Capitol Hill, where two senators said they would try to block it because it could hurt efforts to prevent Iran from getting nuclear weapons.

Sen. Norm Coleman, R-Minn., along with Sen. Evan Bayh, D-Ind., are circulating a letter that will urge Bush not to send the pact to Congress. The senators say Russia's exports of nuclear fuel to Iran's Bushehr nuclear power plant and opposition to the United Nations sanctions against Iran make the new deal suspect.

But the Bush administration now views Russia as a partner in the effort to persuade Iran to abandoned its nuclear weapons ambitions and a State Department official said Russia's assistance to the Iranian power plant was not seen as an issue.

The deal - signed by Burns and Russian atomic energy chief Sergei Kiriyenko - will give the U.S. access to Russian state-of-the art nuclear technology.

That would be important for the Washington, where nuclear development was virtually dormant in the wake of a 1979 reactor accident at Three Mile Island in the U.S. and the 1986 Chernobyl nuclear explosion in the Soviet Union, experts say.

The U.S. is especially interested in developments in areas including fast-neutron reactors and recycling nuclear fuel.

The deal could also help Russia in its efforts to establish an international nuclear fuel storage facility by importing and storing spent fuel. It cannot achieve that goal without signing the deal, since the U.S. controls the vast majority of the world's nuclear fuel.

The fuel storage plans have caused outrage among environmentalists and ordinary Russians, who fear that such a project would turn the country into the world's nuclear dump. Russian officials would have to overcome those objections to go ahead with the plans.

Kiriyenko, meanwhile, insisted that the deal does not mean Russia would be importing nuclear fuel: "Russia is not importing and will not import nuclear fuel," he said.

Work on the agreement got under way after outgoing President Vladimir Putin and U.S. counterpart George W. Bush pledged to increase cooperation in the field at the Group of 8 summit in St. Petersburg in 2006.

The U.S. administration's willingness to reverse course and work with Russia appears to reflect the U.S. view that Moscow is now a partner in the effort to persuade Tehran to abandon nuclear weapons ambitions.

"This is a nod to the long and friendly relations between the Bush and the Putin administration and it sets the stage for some successful nuclear cooperation with the new administrations," in the Kremlin and the White House," said Rose Gottemoeller, the director of the Carnegie Moscow Center.

To get the deal in place, Bush must still send it to Congress. It would become effective unless Congress passes legislation within 90 days to block it. A bill pending in the Senate would block such an agreement unless Russia has stopped cooperating with Iran's nuclear or advanced conventional and missile program, or Iran has stopped enriching uranium.

The U.S. has similar agreements with other major economic powers, including China.