WASHINGTON - The United States and Russia have reached agreement on most major issues in a new treaty that would cut their deployed strategic nuclear warheads by one-fourth and allow each side to continue to verify the other's stockpiles, officials said yesterday.

Negotiators have been racing to conclude a replacement for the 1991 Strategic Arms Reduction Treaty, or START, the nuclear giants' most extensive nuclear treaty. It expired Dec. 5.

President Obama said that the two sides are "quite close" to concluding a new version of the pact.

"We're making excellent progress," Obama told reporters after meeting with Russian President Dmitry Medvedev in Copenhagen during the U.N. meeting on climate change.

"I'm confident that it will be completed in a timely fashion," he added.

However, negotiators are not expected to finish hammering out details, particularly on verification procedures, until at least January.

Obama sees the treaty as key in reestablishing U.S. credibility on arms control in the run-up to an international conference next year reviewing the Nuclear Nonproliferation Treaty.

The U.S. government hopes to tighten the global nuclear regime to discourage countries such as Iran from developing a bomb.

The new treaty is expected to reduce deployed nuclear warheads used for long-range missions from 2,200 to between 1,500 and 1,675.

It also is expected to slash the number of vehicles that can carry nuclear warheads or bombs from 1,600 to about 800. Those vehicles include heavy bombers, intercontinental ballistic missiles and submarines.

The delay in completing the talks before the end of the year was a disappointment to the White House, which had sought a quick boost in its efforts to demonstrate improved relations with Moscow.

The administration had identified a successor to START as among the most achievable areas of cooperation with Russia, as it seeks broader help from Moscow on issues including reining in Iran's suspected nuclear ambitions.