Foreign ministers of the 15 NATO nations today welcomed President Reagan's proposal to cut stockpiles of long-range nuclear missiles as "a far-reaching but realistic offer" that could lead to "fair and effective agreements."

The ministers ended their two-day spring meeting here as Soviet President Leonid I. Brezhnev, speaking in Moscow, replied to President Reagan's nuclear reduction plan by saying his country was willing to begin talks if the United States agrees to a freeze on strategic nuclear weapons.

A communique issued by the NATO foreign ministers today called on Moscow "to respond positively" to Reagan's proposal, made in his May 9 speech at Eureka College in Illinois, for beginning Strategic Arms Reduction Talks (START) by the end of June.

Reports of the Brezhnev speech drew a cautious reaction from Secretary of State Alexander M. Haig Jr., who said he had not seen the full text of Brezhnev's remarks. But, while Haig repeated past U.S. rejections of the freeze idea, he added: "To the extent they're willing to get into the negotiations as early as possible, that's fine."

Haig spoke at a news conference concluding the meeting which was dominated by broad expressions of European support for Reagan's intercontinental missile reductions proposals and concern about the effects on the Atlantic Alliance of Britain's confrontation with Argentina.

In regard to the Falklands crisis, the final communique reiterated opposition to the use of force to resolve disputes and said:

"The allies condemn Argentina for its aggression against the Falkland Islands and deplore the fact that after more than six weeks she still has not withdrawn her forces in compliance with mandatory Resolution 502 of the U.N. Security Council. They call for a continuation of the efforts to achieve a satisfactory negotiated settlement in accordance with this resolution in its entirety."

This tough language gave a new boost of allied backing to Britain which last night found itself hard-pressed to get its partners in the 10-nation European Community to extend their sanctions against Argentina for an additional seven days.

The strains evident in the community over the Falklands issue had been a blow to Britain's hopes of keeping international pressure on Argentina.

Both Haig and NATO Secretary General Joseph Luns said that, during the two days of discussion here, they had not heard a single word of criticism directed against Britain and its military action in the South Atlantic.

On the arms-reduction issue, the ministers endorsed the idea of negotiations "to eliminate totally U.S. and Soviet intermediate-range, land-based missiles and to make substantial reductions in their intercontinental strategic nuclear systems." The communique said: "The allies urge the Soviet Union to respond, without further delay, in a positive way to these proposals which are designed to improve security and achieve a military balance at the lowest possible level of forces."