MOSCOW (AP) -- The United States and Russia reaffirmed on Thursday their 1972 treaty banning missile defense systems and agreed in principle to work on further reducing their nuclear warhead stockpiles.

The agreements came after three days of high-level discussions between the two nations about their nuclear arsenals.

One of the main points the sides discussed was the United States' desire to modify the Anti-Ballistic Missile Treaty so it can build a limited missile defense system.

The system the United States wants to build would be designed to shoot down a single missile or a small number of missiles. It would not be effective against a massive attack, the kind Russia is capable of launching, the Americans say.

Moscow strongly opposes changes to the treaty, saying a missile defense system in the United States would upset the current strategic balance. But President Boris Yeltsin agreed to discuss ABM modifications when he met President Clinton in June.

After this week's talks, the two sides said the ABM treaty "is the cornerstone of strategic stability" between them. No specific proposals were discussed and no major decisions reached, but the sides agreed that ABM must remain strong.

The head of the Russian delegation at the talks repeated Russia's stance that any modifications to the treaty could set off a new arms race.

If the United States builds a missile defense system, "Russia will be forced to raise the effectiveness of its strategic nuclear arms forces and carry out several other military and political steps to guarantee its national security," Grigory Berdennikov, the head of the Foreign Ministry's department for security and disarmament, was quoted by the Interfax news agency as saying.

The sides also agreed in principle to start work on a START III treaty that would cut nuclear warheads to 2,000 to 2,500 per nation.

But before they can work on such a treaty, they have to finish ratification of START II. That treaty, which calls for both countries to scale back from around 6,000 warheads to 3,000 to 3,500 each, was signed in 1993. But Russia's parliament has yet to ratify it.

A Russian lawmaker involved in the talks said START II's ratification depends on a U.S.-Russian agreement on the ABM question.

"The Russian side made it plain that (START II) can be ratified only if there is a mutually acceptable stand" on the ABM treaty, Vladimir Lukin, the chairman of parliament's committee for international affairs, told the ITAR-Tass news agency.

The Kremlin has urged the lower house, the State Duma, to make START II ratification a priority. But Communists and other hard-liners have balked, saying the treaty endangers Russia's security.

The U.S. Senate ratified START II in 1996.