The NATO defense ministers announced here today that the alliance will remove 1,400 of the 6,000 U.S. tactical nuclear weapons now stockpiled in western Europe in a move aimed at defusing current opposition to the introduction of new American medium-range nuclear missiles.

The reductions, however, will be stretched over the next five to six years to mollify critics of the unilateral step.

Today's long-awaited announcement, which affects mostly older weapons, came after four years of study by a NATO group and was timed to coincide with the expected arrival in England next week of a first group of intermediate-range cruise missiles, NATO sources said.

In a statement, the NATO ministers urged the Soviet Union to match yesterday's action by reducing its own buildup of European nuclear forces and said, "Contrary to the impression that NATO has been fueling an arms buildup . . . this sustained program of reductions will have reduced NATO's nuclear stockpile to the lowest level in over 20 years."

The ministers reaffirmed an earlier decision that in addition to the cut of 1,400 warheads announced yesterday they also would drop one additional warhead in the stockpile for each intermediate-range Pershing II and ground-launched cruise missile deployed over the next five years.

Thus if the deployment of the planned 572 Pershing and cruise missiles takes place, the total reductions would come to 1,972.

Defense Secretary Caspar W. Weinberger, in a news conference this afternoon, pointed out that overall, NATO will have reduced the U.S. nuclear stockpile in Europe by one-third if all planned reductions take place.

Pressure for the reductions originated in 1979 with the Netherlands government, which feared public opposition to its agreement then to station 48 U.S. cruise missiles on its territory. The Dutch have yet to accept deployment of those missiles, but the West Germans and British became supporters of the reduction idea as opposition to deployment grew in their country.

The delay in implementing today's announced reductions reflects the opposition of the NATO military staff, according to sources.

NATO Secretary General Joseph Luns said atomic warheads for Nike-Hercules anti-aircraft weapons and nuclear artillery shells were among the kinds of weapons that might be cut.

The Nike-Hercules, however, is to be replaced by a conventional anti-aircraft system called the Patriot. And about 2,000 short-range 8-inch and 155mm artillery shells now deployed in Europe are almost 20 years old. The 8-inch are to be replaced with so-called neutron shells which the United States is building but cannot deploy in Europe because of political opposition. Weinberger yesterday refused to include the 8-inch shells in the reduction or discuss whether the neutrons would replace them in Europe.

Congress recently refused production money for the 155mm neutron shell. Now the Army is planning to request a non-neutron nuclear shell to replace the older one, but it will take years before the first of those will be ready for deployment in Europe.

Meanwhile, the State Department said yesterday that said Soviet President Yuri V. Andropov's latest arms control proposal contains "little new and continues to misrepresent the facts and to attach unacceptable conditions for any agreement" limiting medium-range missiles in Europe and Asia.

The official U.S. commentary also claimed that the presentation of these proposals publicly by Andropov before they were presented at the U.S.-Soviet negotiations in Geneva "shows that the Soviet intention is to try to split the allies and their publics" from the United States rather than negotiate seriously.

Officials here said the one slightly encouraging element of the new Andropov proposal, which came in the form of an interview with the Communist Party newspaper Pravda, was the expressed willingness to limit Soviet medium-range SS20 missiles based in Asia as well as in Europe. The United States has not wanted them limited in Europe only to be moved to Asia.

The Soviet proposal, State said, still would not allow deployment of any new U.S. intermediate-range missiles to match those Moscow already has deployed.

The State Department yesterday disclosed that one new SS20 base with nine missile launchers that had been under construction in the central Asian portions of the U.S.S.R. is operational, bringing the total number of SS20s in Europe and Asia to 360. At least two other such bases are known to be under construction.