The United States is ready to cut the number of nuclear weapons delivery vehicles as part of an agreement with Russia to replace a Cold War arms treaty, Washington's chief negotiator was quoted as saying on Monday.

President Dmitry Medvedev and U.S. President Barack Obama agreed last month in London to pursue a new deal to replace the 1991 START I nuclear disarmament treaty that expires in December.

Russia has said any agreement must limit both warheads and all types of nuclear weapons delivery vehicles — the actual rockets and other means that deliver nuclear weapons — but Washington's position had been unclear.

Rose Gottemoeller, U.S. assistant secretary of state for verification and compliance, told Interfax that Washington was willing to cut both warheads and delivery vehicles.

"In the presidential instructions received after the meetings in London, there is a clear order that negotiations should be focused on strategic offensive weapons, and this covers delivery vehicles and warheads," Gottemoeller said.

Interfax also quoted Gottemoeller as saying the United States was not ready to cut the warheads that have been dismounted from rockets and that are stored in U.S. arsenals.

Russia has insisted on counting all warheads as part of a new deal, but the United States believes that only those deployed on existing missiles should be counted.

Some analysts say the issue of how to count numbers of nuclear warheads could complicate negotiations.

Finding a replacement to the Strategic Arms Reduction Treaty, the largest such treaty in history, is seen by both Moscow and Washington as an opportunity to improve ties that have been badly strained in recent years.