Defense ministers of the North Atlantic Treaty Organization today offered strong support for the new U.S.-Soviet strategic arms limitation pact during informal contacts with U.S. officials here.

They are expected to issue a formal endorsement of the agreement in a communique at the close of the semi-annual NATO ministers' meeting Wednesday.

Support by the ministers from the 13 members of the NATO alliance undoubtedly will be important to the Carter administration as it takes its defense of the proposed SALT II treaty to Capitol Hill.

Sources here said the United States was particularly glad to get quick support for the accord from the newly elected government of Prime Minister Margaret Thatcher in Britain.

Some conservative commentators in various West European newspapers and some political circles have expressed reservations about the treaty-suggesting it will, in the name of detente, make it more difficult to overcome Western defense deficiencies. But European governments generally favor its ratification because failure could cast a severe chill over East-West relations, which would be felt most strongly in day-to-day terms here in central Europe.

The ministers heard a warning today that the Soviet Union is trying to destabilize the nuclear balance of power in Europe by deployment of new medium-range missiles not covered by the SALT agreements.

Gen. H. F. Zeiner Gundersen of Norway, chairman of NATO's military committee, said continuing Soviet deployment of the SS20 mobile missiles - of which about 100 are in place aimed at Western Europe - is the most important military development of the year.

The question of what to do about them along with calls for increased military spending by NATO countries, have dominated NATO discussions for the past year. They will continue to do so for some time, since NATO has pledged to propose specific plans this year on how to counter the new Soviet missiles.

On spending, the defense ministers today in effect extended a pattern set in 1977 when, responding to President Carter's initiative, they agreed to increase defense expenditure by 3 percent annually. This covered the 1979-1983 period and today's action extends this to 1985.

In fiscal years 1979 and 1980, U.S. officials say, NATO did average about 3 percent, mostly on the strength of the big U.S. and West German defense budgets. Turkey and Portugal, NATO countries with severe economic problems, made known their inability to comply, but officials here said there was no other objection to the extended goals.

The ministers also endorsed a new five-year, $4.5 billion NATO "infrastructure" program. This is a common fund that supports common projects within the alliance. Many involve building more shelters for warplanes, ammunition dumps, runways and other facilities.

Though the approved program represents about a one-third hike over the last five-year plan when inflation is included, it is still about one-third below what the United States was pressing for. U.S. officials indicated the West Germans were among those eager to keep spending lower. This, the Germans said, is not only for financial reasons, but also because Bonn would like to see the United States spread its facilities around more to other countries in the alliance.

Although no major new decisions are expected here on the Soviet intermediate-range missile question, the problem-which is the most complex and potentially explosive politically to confront NATO in many years-is never out of mind.

For example, U. S. Defense Secretary Harold Brown today met privately with Belgian Defense Minister Paul Vanden Boeynants. The U. S. defense chief reportedly pressed his counterpart on the need for the widest possible alliance participation in any future decision to place new tactical nuclear weapons in Europe to balance the Soviet SS20s and other Soviet weapons.

A major problem is West Germany. The Bonn government has made it clear that while it will accept an alliance decision to deploy new U.S.-controlled atomic weapons, it will not allow itself to be the only country on whose soil those new weapons are deployed. Furthermore, Bonn has made it clear that since it is not a nuclear power, it will not consider acceptance by Britain, for example, which is a nuclear power, to be enough company on this question. Bonn wants nonnuclear countries such as Holland, Belgium or Italy also to accept these new weapons.

One possibility being discussed is to present the NATO response to the Soviet weapons as part of a long-term project in which the first likely new counter-weapons-the U.S.-built extended-range Pershing missile, which could be ready in one year-would be based in West Germany, while newer weapons such as longer-range land-and sea-based missiles and cruise missiles that would be available later would be based elsewhere.

Whatever happens, Bonn is certain to face a sharp political debate at home because even with an alliance decision many West Germans would be opposed to new atomic-tipped missiles on West German soil.

Although U. S. atomic weapons have been on West German soil for many years, these new ones would be capable of reaching Soviet territory for the first time. Bonn, wishing to maintain its ties with the alliance without making itself a target for Soviet propaganda that could harm its improved relations with Moscow, is sticking thus far to these conditions.

U. S. officials say privately they understand and agree with the West German conditions and thus are pressing at every opportunity to get some of the smaller countries to go along with any future NATO decision. The political situation in the Netherlands, Belgium and Italy, however, is such that it will be very difficult to get agreement. CAPTION: Picture, HAROLD BROWN . . . meets Belgian colleague