US President Barack Obama and Russian President Dmitry Medvedev have agreed a new nuclear arms reduction treaty after months of negotiations.
The treaty limits both sides to 1,550 warheads, about 30% less than currently allowed, the White House said. The deal replaces the 1991 Strategic Arms Reduction Treaty. The leaders will sign the pact in Prague on 8 April.
President Obama hailed the treaty as the most comprehensive weapons control agreement in nearly two decades.
"With this agreement, the United States and Russia - the two largest nuclear powers in the world - also send a clear signal that we intend to lead," he said at the White House.
"By upholding our own commitments under the Nuclear Non-Proliferation Treaty, we strengthen our global efforts to stop the spread of these weapons, and to ensure that other nations meet their own responsibilities," he said.
In Russia, President Medvedev's spokeswoman told the Interfax news agency: "This treaty reflects the balance of interests of both nations."
The treaty must be ratified by the US Senate and the Russian Duma.
New limits
In a speech in Prague last April, Mr Obama set out his vision of moving towards a world without nuclear weapons.
Both sides agreed to cut their arsenals last year, but disagreements on verification have held up a deal.
The US is said to have more than 2,000 deployed strategic nuclear weapons, while Russia is believed to have more than 2,500.
The new agreement - which came in a phone call between the two leaders - limits the US and Russia to a maximum of 1,550 deployed nuclear warheads each.
The cuts are substantial - well over 30% for the Russians and around 25% for the Americans, whose current arsenal is smaller, says a news channel diplomatic correspondent James Robbins.
Both sides would have seven years after the treaty's ratification to carry out the reduction in long-range nuclear warheads.
The agreement also calls for cutting by about half the missiles and bombers that carry the weapons to their targets.
It limits missile delivery vehicles to 800 deployed and non-deployed intercontinental ballistic missile launchers, submarine-launched ballistic missile launchers, and heavy bombers equipped for nuclear weapons.
The cap on deployed intercontinental ballistic missiles and submarine launched missiles is set at 700, the White House said.
The agreement includes a new verification mechanism that will ensure the "irreversibility, verifiability and transparency" of the reduction process, Russia's Itar-Tass news agency said.
Moral high ground
The pact establishes a "legally-binding" linkage between offensive weapons and missile defence systems, the Kremlin said in a statement, and "will demand the deployment of all strategic offensive weapons exclusively on national territories".
Moscow has strongly opposed US plans to set up missile defences in Europe, and has insisted on explicit recognition of the link between offensive and defensive systems in any new strategic arms reduction pact.
The timing and symbolism of the deal are crucial, enabling both countries to claim some moral high ground going into next month's Washington Summit on nuclear security, and the critical talks in May aimed at limiting the spread of nuclear weapons around the world, our correspondent says.
Presidents Obama and Medvedev hope the new deal will increase pressure on Iran, in particular, to abandon any ambition to develop nuclear weapons, he adds.
The agreement - called the Measures to Further Reduction and Limitation of Strategic Offensive Arms - replaces the Cold War-era Start treaty signed in 1991 and the Moscow Treaty signed in 2002.
Both US and Russian officials expressed confidence that lawmakers would ratify the treaty.
