WASHINGTON (Reuters) - The United States will soon announce plans to suspend compliance with a landmark nuclear missile pact with Russia, responding to an alleged violation of the treaty by Moscow, U.S. officials said on Thursday.

The move would start a six-month countdown that could lead to permanent U.S. withdrawal from the 1987 arms control accord, which bans either side from stationing short- and intermediate-range, land-based missiles in Europe, the officials said.

However, Washington could choose not to pull out of the Intermediate-range Nuclear Forces Treaty (INF), if Russia comes into compliance with the treaty within that time frame.

The United States alleges that a new Russian cruise missile violates the pact. The missile, the Novator 9M729, is known as the SSC-8 by the North Atlantic Treaty Organization (NATO).

Russia denies the allegation, saying the missile’s range puts it outside the treaty, and has accused the United States of inventing a false pretext to exit a treaty Washington wants to leave anyway so it can develop new missiles. Russia has also rejected a U.S. demand to destroy the new missile.

“We’re going to announce suspension,” a U.S. official told Reuters on condition of anonymity .

A second U.S. official said the U.S. action would be “reversible” if Russia came back into compliance during the six-month U.S. suspension. “Then the U.S. would unsuspend,” the official said.

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The dispute is aggravating the worst U.S.-Russia frictions since the Cold War ended in 1991. Some experts believe the treaty’s collapse could undermine other arms control agreements and speed an erosion of the global system designed to block the spread of nuclear arms.

U.S. Under Secretary of State for Arms Control and International Security Andrea Thompson on Thursday held last-ditch talks with Russian Deputy Foreign Minister Sergei Ryabkov in Beijing ahead of the expiration on Saturday of a U.S. 60-day deadline for Moscow to return to compliance with the treaty.

Thompson and Ryabkov said afterwards that the two countries had failed to bridge their differences. They met on the sidelines of a meeting of the U.N. Security Council’s five permanent members - the United States, Russia, China, France and Britain - all nuclear powers.

European officials are concerned about the treaty’s possible collapse, fearful that Europe could again become an arena for nuclear-armed, intermediate-range missile buildups by the United States and Russia.

In an interview, Thompson said she expected Washington to stop complying with the treaty as soon as this weekend, a move she said would allow the U.S. military to immediately begin developing its own longer-range missiles if it chose to do so, raising the prospect they could be deployed in Europe.

“We’ll be able to do that (suspend our treaty obligations) on Feb. 2,” she told Reuters in Beijing. “We’ll have an announcement made, follow all the steps that need to be taken on the treaty to suspend our obligations with the intent to withdraw.”

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Once announced, the formal withdrawal process takes six months. Halting treaty compliance would untie the U.S. military’s hands, Thompson said.

“We are then also able to conduct the R&D and work on the systems we haven’t been able to use because we’ve been in compliance with the treaty,” she said. “Come February 2, this weekend, if DoD (the U.S. Department of Defense) chooses to do that, they’ll be able to do that.”

Washington remained open to further talks with Moscow about the treaty, she added.

Ryabkov said Moscow would continue working toward an agreement but accused Washington of ignoring Russian complaints about U.S. missiles and of adopting what he called a destructive position.

“The United States imposed a 60-day period during which we had to fulfill their ultimatum,” the Sputnik news agency quoted Ryabkov as saying after talks with Thompson. “I conclude that the United States was not expecting any decision and all this was a game made to cover their domestic decision to withdraw from the INF Treaty.”

Daryl Kimball, executive director of the Arms Control Association, a Washington think tank, said he sees little chance the treaty can be saved.

“Neither side is showing the flexibility necessary to arrive with an agreement that brings Russia into compliance,” he said. “So I think it’s highly unlikely that we will see an 11th hour diplomatic miracle.

“Both sides at this point appear more interested in winning the blame game than taking the steps necessary to save the treaty,” he added.