U.S. Secretary of Defense Caspar Weinberger declared today that the Reagan administration "will make every effort to resist" any congressional initiatives that would link the continued presence of more than 300,000 American troops in Europe to expanded nonnuclear defense commitments by the European allies.

Speaking at a press conference following a two-day meeting of North Atlantic Treaty Organization defense ministers, Weinberger also said the Soviet Union has increased its arsenal of SS20 missiles by nine, to 387, and is constructing sites that could accommodate at least 100 more of the triple-warhead rockets.

In the absence of an arms control pact limiting medium-range missiles in Europe, the 14 ministers emphasized in a communique NATO's determination to stick to the deployment schedule of 572 Pershing II and cruise missiles to counter the SS20 buildup.

Ministers from the Netherlands and Belgium endorsed the alliance position although they face grave political problems within their ruling coalitions over the cruise missiles. Only Greece and Denmark, countries that will not receive any missiles, expressed reservations.

Weinberger expressed satisfaction with the decision by the ministers to approve nearly $8 billion over the next six years to upgrade military ground facilities and ammunition stocks in order to enhance the U.S. ability to reinforce Europe during a military crisis.

He said he found such actions "enormously encouraging" as evidence that Europe was willing to do its share in defending the continent and that he would seek to convey that message to congressional skeptics.

Last June, Sen. Sam Nunn (D-Ga.) proposed an amendment calling for U.S. forces in Europe to be reduced by one-third unless European countries took more responsibility for their own defense. The amendment was narrowly defeated, but U.S. and European officials here said they expected similar legislation to be revived in the new Congress.

"It is critically important that NATO be supported and that we recognize that we in the United States could not live in a world that was overrun by Soviet hordes," Weinberger said.

Several European ministers expressed resentment over the congressional pressure to extract more defense spending from the allies at a time when Europe's economies continue to be troubled by unemployment and low growth.

"We do not need any American senator to tell us where deficiencies in our conventional forces may be," said West German Defense Minister Manfred Woerner.

Britain's Michael Heseltine said that while his country and other European members of NATO were prepared to rectify shortcomings in their defenses, they could not be forced to allocate more resources beyond current plans because of austerity budgets.

While stressing their readiness to respond to the Soviet threat by strengthening NATO's nuclear and conventional capabilities, the ministers also welcomed coming talks between Moscow and Washington "with the objective of reaching mutually acceptable agreements on the whole range of questions concerning nuclear and outer-space arms."

Weinberger briefed his colleagues today on U.S. preparations for next month's meeting in Geneva between Secretary of State George P. Shultz and Soviet Foreign Minister Andrei Gromyko. That session is supposed to produce an agenda for future arms control talks.

Both Weinberger and NATO Secretary General Lord Carrington said there was recognition among the allies that the Geneva meeting should not raise "false expectations" since it is only the start of what are likely to be long and arduous negotiations.

Weinberger's report of an SS20 buildup raised perplexing questions about the future of Soviet nuclear strategy, NATO officials said.

They said the Soviet Union is believed to have activated four new sites of nine missiles each in recent months but to have withdrawn SS20s from three other sites, leaving a net increase of nine missiles from NATO's previous estimate of 378.

NATO sources said there is strong speculation that the Soviets are changing some SS20 sites to take a new mobile intercontinental missile called the SS25.

The number of SS20s deployed is considered significant for NATO's deployment schedule because the Dutch government has decided to accept its share of 48 new cruise missiles next November only if there is no arms control agreement and the Soviet arsenal has surpassed the threshold of 378 missiles.

But a Dutch official said his government will face a new dilemma next year if so many SS20 launchers are converted to take the longer-range SS25 rocket that the number of SS20s falls below 378. In that case, the Netherlands has said it would not deploy any cruise missiles.