WASHINGTON — President Barack Obama and Russian President Dmitry Medvedev agreed today to sharp cuts in the nuclear arsenals of both nations in the most comprehensive arms control treaty in two decades. "We have turned words into action," Obama declared.

Obama said the pact, to be signed April 8 in Prague, was part of his effort to "reset" relations with Russia and a step on a path toward "the peace and security of a world without nuclear weapons."

The agreement would require both sides to reduce their arsenals of long-range nuclear weapons by about one-third, from 2,200 now to 1,500 each. The pact, replacing and expanding a 1991 treaty that expired in December, was a gesture toward improved U.S.-Russian relations that have been badly frayed.

The reductions would still leave both sides with immense arsenals — and the ability to easily annihilate each other.

"In many ways, nuclear weapons represent both the darkest days of the Cold War, and the most troubling threats of our time. Today, we have taken another step forward in leaving behind the legacy of the 20th century while building a more secure future for our children," Obama said at the White House.

In Russia, Medvedev's spokeswoman Natalya Timakova told the Interfax news agency, "This treaty reflects the balance of interests of both nations."

Both sides would have seven years after the treaty's ratification to carry out the about 30 percent reduction in long-range nuclear weapons. The agreement also calls for smaller cuts to warheads and bombs based on planes, ships and land.