"The Russian Federation notes the absence of any preconditions for START II to come into force and no longer considers itself committed to the international legal obligations" of the treaty, the Russian Foreign Ministry said in a statement.

START II, which was signed in 1993, would have reduced nuclear warheads to 3,000 to 3,500 each, but the treaty was never implemented and has been considered dead for some time.

The U.S. Congress ratified the treaty in 1996 and the Russian parliament followed suit in 2000, but Russian lawmakers linked START II to preservation of the 1972 Anti-Ballistic Missile Treaty.

The U.S. Congress never accepted the Russian conditions, and the United States announced it would withdraw from the ABM treaty in order to pursue a national missile defense system, which was prohibited under ABM rules.

The ABM treaty expired on Thursday.

Russia has strongly criticized the United States for pulling out of the treaty, which it called a cornerstone of global security. But Russian President Vladimir Putin and U.S. President George W. Bush signed a new nuclear arms reduction pact in May, as well as a declaration of partnership in which they pledged to exchange information about missile defense programs and explore possibilities for cooperation in missile defense.

The new arms reduction agreement reduces strategic nuclear warheads to 1,700 to 2,200 for each country, down from 6,000 or more for the United States and about 5,500 for Russia. It also gives the two sides more flexibility to decide which weapons they will scrap.

The START II treaty specifically banned Russia from deploying land-based missiles with multiple warheads — an arsenal that formed the core of its nuclear forces. With START II in effect, Russia would have had to deploy a large number of new single-warhead Topol-M missiles or build nuclear submarines equipped with ballistic missiles to match U.S. arsenals. The government can't afford either option.

Following the new treaty, Russia can keep its Soviet-built multiwarhead SS-18 and SS-19 missiles for a while, avoiding a costly race to build a replacement. But these older weapons are already slated to be scrapped this decade.

Russia also may try to maintain nuclear parity with the United States by fitting its new Topol-M missiles with three warheads each.