The Bush administration will inform Russia Monday that it is prepared to press ahead with a unilateral withdrawal from the 1972 Anti-Ballistic Missile Treaty to build a missile defense system, according to a senior administration official.

Seeking to put to rest questions about whether President Bush will still make missile defense a priority after last week's terrorist attacks in the United States, the administration plans to tell Russian in talks Monday that, "if anything, the likelihood of unilateral withdrawal has increased" as a result of the attacks at the World Trade Center and the Pentagon, said the U.S. official.

"Missile defense will not fade as a priority of the administration. These incidents prove that there are people in the world for whom the concept of deterrence doesn't mean a thing," the official said, speaking on condition of anonymity. "This was high-tech terrorism; these people had jet plane pilots. And if these same people had access to ballistic missiles, do you think they wouldn't have used them?"

Undersecretary of State John Bolton arrived in Moscow today for the talks, which were to be held in London last week but were postponed after the attacks. He is to meet with Deputy Foreign Minister Georgy Mamedov Monday, and sources said the two would also likely discuss potential Russian cooperation with U.S.-led retaliatory strikes following the attacks.

In recent weeks, top Russian officials have signaled a newfound willingness to accept U.S. withdrawal from the ABM Treaty, a move they previously said would be tantamount to unraveling "the entire framework of international security."

Indeed, just minutes before the planes crashed into the World Trade Center last Tuesday, a Russian general told reporters here that a U.S. withdrawal would not affect the "level of trust" between the two countries and that Russia was prepared to negotiate a new, post-Cold War security structure even after such a move.

Now, according to the Bush administration official, "the Russians have come to an acceptance that, absent some major development, the United States is going to withdraw from ABM unilaterally or at least give notice of withdrawal. They have realized that maybe we're not going to negotiate on this before the treaty is gone."

But last week's attacks by knife-wielding terrorists have also sparked a new round of public criticism of Bush's missile defense plans here. Many top Russian officials have gone out of their way to point out the relatively low-tech nature of the attacks, insisting that it undermines Washington's stated reason for spending billions of dollars on a system of missile defense aimed at heading off a nuclear attack by small, hostile states such as North Korea.

On Wednesday, Russian Foreign Minister Igor Ivanov is due in Washington for talks on missile defense with Secretary of State Colin L. Powell, and U.S. sources said they are expecting new proposals from the Russians then. At the same time, in Moscow, U.S. Deputy Secretary of State Richard Armitage is slated for meetings on enlisting Russia in the anti-terrorism fight, and specifically on what support Russia can provide for possible U.S. strikes in Afghanistan.

Since last week's attacks, Russian leaders have pledged support for the United States but have ruled out Russian participation in military strikes.