After a long impasse, President Clinton and Russian President Boris Yeltsin have agreed to make a fresh attempt to resolve contentious treaties on strategic nuclear weapons and anti-ballistic missile defenses.

In a joint statement reached at their weekend summit in Cologne, Germany, both presidents indicated a new willingness to take small negotiating steps they had eschewed.

Yeltsin said Russia is prepared to listen to American proposals for amending the 1972 Anti-Ballistic Missile Treaty, although he told Clinton that Russia remains strongly opposed to any changes, the Interfax news agency reported. Clinton said the United States is ready to proceed with discussions about a START III arms reduction treaty even though its forerunner, START II, remains unratified by the Russian parliament.

The START II treaty was on the verge of ratification by the lower house of the Russian parliament, the State Duma, when NATO unleashed airstrikes against Yugoslavia in March, creating a wave of anti-American sentiment that dashed hopes for approval. Russia's sympathy with the Serbs, and anger that NATO acted without U.N. Security Council authorization, also led the Kremlin to suspend arms control talks with the United States.

The White House now expects Deputy Secretary of State Strobe Talbott to lead a U.S. delegation in two-track negotiations in late summer. One track will be the U.S. desire to modify the ABM treaty to allow for the possible construction of a national missile defense system. In an initial meeting in February, the U.S. side stressed that the missile defense system would be aimed at shooting down a small number of missiles launched by a rogue state and would not be robust enough to counter all of Moscow's missiles.

The other negotiating track would resume dialogue in preparation for a START III treaty. Clinton and Yeltsin set tentative limits on warheads for the prospective treaty at a March 1997 meeting in Helsinki. Those ceilings would reduce the number of warheads on each side to between 2,000 and 2,500, from the 3,000 to 3,500 allowed by START II.

Previously, the Clinton administration had insisted that it would not begin formal negotiations on the next strategic arms treaty until START II was ratified. The joint statement pledges that "discussions" will begin later this summer on START III, while both sides promised to "do everything in their power" to win ratification of START II.

The START II treaty was signed in January 1993 by Yeltsin and President Bush, and was approved by the Senate in 1996. It has languished in the Duma, which is dominated by Communists and nationalists.

Yeltsin has repeatedly promised to win ratification of the treaty but has not followed through. The Duma is scheduled this week to break for the summer and is not planning to take up the treaty before departing. In the autumn, the chamber will be preoccupied with reelection campaigns, making the outlook for START II cloudy, at best -- although the speaker of the Duma, Gennady Seleznev, has said the pact will be on the agenda.

Despite the delays in ratification, Russia's strategic nuclear arsenal is already shrinking below START II levels as older weapons become obsolete and the Kremlin lacks money to build new ones. Most Russian experts now say Moscow cannot afford to maintain the START II levels and may not even be able to reach the proposed START III ceilings due to the retirement of submarines, missiles and airplanes that carry nuclear weapons.

Staff writer Walter Pincus contributed to this report.

CAPTION: President Clinton and Russian President Boris Yeltsin meet before the summit in Cologne, Germany, where the leaders agreed to fresh effort on arms talks.