A White House official said Wednesday that the United States and Russia are "very close" to an agreement on a nuclear arms reduction treaty.  The official says that when a signing ceremony is held, it will be in Prague.

White House Press Secretary Robert Gibbs says the two countries are working out details of an agreement to replace the 1991 Strategic Arms Reduction Treaty, or START, that expired in December.

Gibbs says President Barack Obama and Russian president, Dmitri Medvedev, will discuss the new treaty by telephone during the next few days.

"We are, I think, very close to having an agreement on a START treaty, but will not have one until President Obama and his counterpart, Mr. Medvedev, have a chance to speak again," said Robert Gibbs.

The Czech Republic said Wednesday that it has agreed to host a signing ceremony for the treaty in Prague.  No date has been set.  

The White House spokesman says the Czech capital has significance for Mr. Obama, who spoke there last April about his vision for a nuclear weapons-free world.

"The new START treaty begins to take many important steps between the two greatest holders of those nuclear weapons, so I would anticipate that when we have something to sign, it will be in Prague," he said.

An agreement would need to be ratified by the Russian parliament and the U.S. Senate before it becomes binding.

President Obama on Wednesday briefed Senators John Kerry, chairman of the Senate Foreign Relations Committee, and Richard Lugar, the panel's top Republican, about the agreement.  Both men would have key roles in getting the accord ratified.

At Wednesday's White House briefing, reporters asked spokesman Robert Gibbs how a new START treaty might affect other aspects of the U.S.-Russia relationship.

"We have been focused on a new type of dialogue and a partnership, where the two countries can address issues of mutual agreement," said Gibbs. "We have worked with them on our next steps on Iran.  We have worked with them in different avenues relating to North Korea."

The previous strategic arms reduction treaty was signed by U.S. President George H.W. Bush and Soviet President Mikhail Gorbachev in 1991, only months before the Soviet Union dissolved.  The pact required each country to reduce its nuclear warheads to about 6,000.

Presidents Obama and Medvedev agreed last July to cut the number of their countries' warheads to about one-fourth of that number within seven years.