MOSCOW (Reuters) - Russia and the United States said on Wednesday they had slashed their strategic weapons stockpile down to the levels required by the START-1 treaty, signed by the United States and the Soviet Union in 1991.

A Russian Foreign Ministry statement said Russia had cut the number of vehicles to 1,136 and the number of nuclear warheads to 5,518, well below the ceilings of 1,600 and 6,000 established by the Strategic Arms Limitation Treaty, which set Wednesday as the deadline for compliance with the targets.

"The United States and Russia each now maintain fewer than the treaty's mandated limits of 1,600 deployed strategic delivery vehicles and 6,000 accountable warheads," U.S. State Department spokesman Philip Reeker added in Washington.

"We're marking an important milestone today in dismantling the legacy of the Cold War. The treaty's final ceilings came into effect today and they have been met," he told a briefing.

The treaty was signed by former President George Bush, father of the current U.S. president, and Soviet leader Mikhail Gorbachev.

The Russian statement said: "The full and timely fulfilment of the provisions in the START-1 treaty establishes good conditions for working out an agreement on further radical reductions of strategic offensive weapons."

The United States added in a written statement: "As we cooperate in building this new strategic relationship and as we move beyond the Anti-Ballistic Missile (ABM) treaty, we will make further reductions in strategic nuclear forces."

Russian President Vladimir Putin and U.S. President George W. Bush announced substantial cuts in nuclear arms stockpiles during Putin's visit to the United States last month, bringing them to the lowest level since the 1950s.

At the summit, Bush announced plans to cut U.S. strategic offensive weapons from 7,000 warheads to between 1,700 and 2,200. Russia has said it is ready to cut the number of its strategic warheads to about 1,500.

However, Russia has complained that Washington has so far failed to make clear whether the United States intends to destroy nuclear warheads or simply remove them from their delivery vehicle and store them.

The Bush administration does not want to enshrine the next phase of reductions in a formal verifiable agreement, arguing that it is enough for each to tell the other of its plans.

The two countries have also failed to agree on the future of the 1972 ABM treaty, which the United States sees as a relic of the Cold War and an obstacle to the missile defence tests it wants to carry out.

Russia still sees the treaty, which bans large-scale missile defences, as a cornerstone of strategic stability.

The Arms Control Association, an independent organization which favours arms control agreements, congratulated Moscow and Washington on the cuts but urged the United States to account for the large and growing stockpile "hedge" warheads which START-1 allows the parties to hold.

"This reserve of some 4,500 to 5,000 strategic and tactical warheads was once mostly intended to provide the United States with the capability to quickly reverse reductions of its deployed arsenal to guard against a Russian buildup," it said.

"Now the presence of the hedge arsenal creates a strong disincentive for Russia to implement further nuclear reductions," the executive director of the association, Daryl Kimball, said in a statement.

"Bush's handshake brand of unilateral, voluntary arms restraint would not only make nuclear stockpiles more opaque and more difficult to verify gut also would do little to decrease the overall size of nuclear stockpiles," he added.