U.S. President Obama and Russian President Dmitry Medvedev on Thursday signed a major nuclear arms control agreement that reduces the nuclear stockpiles of both nations.

The new Strategic Arms Reduction Treaty - known by its acronym, START- builds on a previous agreement that expired in December.

The agreement cuts the number of nuclear weapons held by the United States and Russia by about a third.

"This day demonstrates the determination of the United States and Russia -- the two nations that hold over 90 percent of the world's nuclear weapons - to pursue responsible global leadership," Obama said after the signing. "Together, we are keeping our commitments under the Nuclear Non-Proliferation Treaty, which must be the foundation for global nonproliferation."

Medvedev called START a "win-win situation" for the two countries. "This agreement enhances strategic ability and, at the same time, allows us to rise to a higher level of cooperation between Russia and the United States," he said, CNN reported.