PRAGUE -- Casting aside years of rancor, President Barack Obama and Russian President Dmitry Medvedev on Thursday signed the biggest nuclear arms pact in a generation, a move that would shrink the Cold War superpowers' arsenals to the lowest point since the frightening arms race of the 1960s.

"This ceremony is a testament to the truth that old adversaries can forge new partnerships," Obama declared. "It is just one step on a longer journey."

The warheads covered by the treaty are lethal relics of the Cold War, and even with the planned reductions there will be enough firepower on each side to devastate the world many times over.

And of more immediate concern are attempts by terrorist groups such as al-Qaida and nations such as Iran and North Korea to acquire or use nuclear weapons.

Obama and Medvedev showed solidarity for a spring showdown with Iran. And, beginning Monday, leaders of 47 countries will gather in Washington in an effort to prevent the spread of nuclear weapons, crack down on illicit nuclear trafficking and lock down vulnerable nuclear materials around the world.

Introduced Thursday with trumpet fanfare, the two grinning presidents sat at an ornate table in Prague's hilltop presidential castle and put their signatures to a landmark successor to the 1991 Strategic Arms Reduction Treaty.

Nearly a year in the making, the "New START" signaled a bold opening in previously soured U.S.-Russia relations. If ratified by both nations' legislatures, it will shrink the limit of nuclear warheads to 1,550 each over seven years, down about a third from the current ceiling of 2,200.

Ratification in the U.S. Senate hardly will be automatic, requiring 67 votes in the 100-member chamber during a congressional election year when cooperation can be hard to come by.

Beyond that, urgent international nuclear tasks still face the two leaders.

For example, they are trying to forge agreement among themselves and four other nations -- China, France, Britain and Germany -- on how to tackle Iran's continued defiance of United Nations demands that it cease enriching uranium.

The West insists that Tehran seeks to develop nuclear weapons; Tehran has said it is after peaceful nuclear power.

At Obama's side, Medvedev made Russia's support for considering a fourth round of U.N. sanctions on Iran clearer than ever. "We cannot turn a blind eye to this," he said of Tehran's intransigence.

But that was not the main question heading into the leaders' talks, which ran overtime to about two hours.

At issue is how weak any new sanctions would need to be to get Moscow on board, not to mention China, an even more stubborn holdout.

Medvedev said sanctions should be "smart," designed to change behavior, not to bring down the hardline Iranian government or impose hardship on Iran's people. The Russian leader said he had outlined for Obama "our limits for such sanctions," and Obama Russia expert Mike McFaul said those discussions got very specific.

"In all negotiations, people talk about their red lines and their bottom lines and we negotiate," McFaul said. White House officials would not reveal details of the private conversation.

Russia's deputy foreign minister, Sergei Ryabkov, said that a total embargo on refined petroleum products into Iran, which depends heavily on such imports, was out of the question for Moscow.

Obama repeated his flat declaration that "strong, tough sanctions" will be agreed to this spring. He said "we will not tolerate" any actions by Iran that risk a new arms race in the Middle East or threaten the security of the international community.

The president faces another key test in that drive when he meets Chinese President Hu Jintao in Washington on the sidelines of Monday's 47-nation summit.

Obama said the U.S. wants to get started on more arms- control negotiations with Russia, seeking larger cuts and ones that target short-range nuclear weapons as well as those held in reserve and in storage. None of those are affected by New START.