The Clinton administration has offered to help Russia complete a key radar site and to share more American radar data if Russia agrees to renegotiate the Anti-Ballistic Missile Treaty so that the United States could build a national missile defense system, senior U.S. officials said yesterday.

In a project that would cost tens of millions of dollars, the United States would help Russia finish a partially constructed radar near the Siberian city of Irkutsk that is oriented eastward, covering northern Asia, including North Korea, and parts of the North Pole. Russia might also be given access to data from U.S. early-warning radars on the full trajectory of missile launches, and the two countries might collaborate on some satellite systems.

Together with the Senate's defeat last week of a treaty banning nuclear test explosions, the attempt to modify the 27-year-old ABM Treaty is a sign of tremendous ferment in the realm of arms control.

The collapse of the Soviet Union, the advent of new technology and the rise of missile threats from countries such as North Korea, Iraq and Iran are pushing Cold War-era agreements toward obsolescence. Wary of tearing up the entire quilt of agreements that took decades to negotiate, however, the Clinton administration is trying to keep Russia as a partner in developing a system to shoot down missiles.

"We've raised with them a number of cooperative activities to show that we see this as a threat that affects both countries," said a senior administration official. "We don't see this as anything against Russia, and we're willing to look at a whole range of cooperative measures that would address the same rogue threat we're concerned about."

The offer, made more than a month ago and first reported in the early editions of today's New York Times, is consistent with earlier statements the administration has made about finding ways to win Moscow's support for national missile defense. Such a system, officials say, would provide a limited defense oriented primarily toward rogue states, not Russia.

Although the Russian government has officially rejected U.S. proposals to renegotiate the ABM Treaty, it has nonetheless agreed to listen to American ideas. The latest round of talks took place last week, and the United States has not yet received a Russian response.

To improve the chances that the Russians will go along, the administration decided last month to ask initially for modest changes in the ABM Treaty, rather than seeking wholesale revisions, as some Republicans in Congress have advocated.

U.S. negotiators are trying to convince Russian counterparts they have a common interest in guarding against rogue states that may soon be able to fire intercontinental missiles at either Russia or the United States. The United States believes it would benefit from Russian radar data covering countries such as Iran and North Korea, and believes it can offer valuable information in return.

"We've been doing a lot of very blue-sky thinking about what kinds of cooperation might conceivably be possible," another senior administration official said. "We've told the Russians we're prepared to be pretty far-reaching in cooperation."

The United States has given the Russians a list of several potential forms of cooperation. In addition to completing the radar at Mishelevka, near Irkutsk, the administration has offered joint computer simulations of antimissile systems; expanded intelligence sharing on threats from rogue states; collaboration in developing two missile observation satellites; a joint presence at one U.S. and one Russian radar site; and joint exercises in battlefield missile defense.

The United States also suggested expanding an agreement reached a year ago to share U.S. radar data on the point of origin and expected destination of missiles. Now, the administration is offering to share information on the entire trajectory.

Another possibility, which remains a subject of intense debate inside the administration, would involve offering to help Russia regain use of a radar in Lyaki, Azerbaijan, that covers some Middle Eastern nations.

"We're not at the point where we have substantial feedback from the Russians that any of these proposals might bear fruit," a U.S. official said.

The American negotiators have cited conflicts in the Muslim republics and territories along Russia's southern border as one reason why Russia should be interested in guarding against launches.

Talks about sharing data on missile launches began late in the Bush presidency. But the issue has taken on greater urgency because of American plans for moving ahead with national missile defense.

The Clinton administration has said it will decide next June whether to proceed with the first phase of the system, consisting of 100 missile interceptors based in Alaska. The second phase would involve a site in the continental United States and a total of more than 200 interceptors.

One of Russia's concerns, according to an administration official, is the possibility the system might be expanded. At the moment, it will be a challenge for U.S. technology to intercept even a small number of missiles, and impossible to shoot down the hundreds of missiles that Russia might launch at once. Thus, the U.S. negotiators argue, the missile defense system would not decrease Russia's nuclear deterrence against the United States.

Russian negotiators, however, want assurances against a "breakout in capacity from limited to something much bigger" that would be capable of knocking down scores of missiles, the administration official said.