The United States and Russia started their third round talks on a new strategic arms reduction pact on Monday in Geneva.

The talks are being held behind closed doors at the U.S. mission in Geneva, a Russian diplomat said Monday.

The talks aim at replacing the Strategic Arms Reduction Treaty known as START-1, which expires in this December.

The START-1 treaty was negotiated in the 1980s, signed by U.S. President George Herbert Walker Bush and Soviet President Mikhail Gorbachev in 1991, and came into force in 1994.

The U.S. team of negotiators is led by Assistant Secretary of State Rose Gottemoeller, while the Russian delegation is headed by Anatoly Antonov, director of the Foreign Ministry's Department of Security and Disarmament.

The first two round talks were held on May 19-20 in Moscow and on June 1-3 in Geneva.

In 2002, a follow-up agreement on strategic offensive arms reduction was concluded in Moscow. The agreement, known as the Moscow Treaty, envisioned cuts to 1,700-2,200 warheads by December 2012.

According to a report published by the U.S. State Department in April, as of January 1 Russia had 3,909 nuclear warheads and 814 delivery vehicles and the United States had 5,576 warheads and 1,198 delivery vehicles.

The talks will continue until June 24.