Defense ministers of the North Atlantic Treaty Organization formally approved the largest single project in alliance history today, but concern over the future level of U.S. defense spending dominated the thoughs of officials from 13 countries gathered here for a two-day semiannual meeting.

Senior U.S. and NATO officials said privately that almost all Allied ministers indicated that if the United States, even for valid and understandable domestic budgest reasons, fails to increase in real defense spending, they would have a difficult and perhaps impossible time getting parliamentary approval for similar increases in their own countries.

It was because of an intiative by Presidetn Carter in 1977 that most of the NATA countries formally, although in some cases reluctantly, agreed to strive for an after-inflation increase of 3 percent each year in their defense budgets for the next five years.

In recent weeks, however, the President, under pressure to increase speding on social programs and cut the budget deficit, appears to be wavering from that goal. This is causing among European leaders because such increases also are politically risky in their own countries especially if the United States backs down.

Defense Secretary Harold Brown, at a press conference at the close of the meeting, acknowledged that his fellow ministers "expressed concern."

"What we do is clearly very important to them, not only in substantive terms but in symbolic terms," he said. "That was made clear to me."

Brown said he told the ministers the same thing he was telling the press , that the president is reviewing the budget carefully but has not yet made up his mind on the final figure.

Privately, several top officials here said they personally feel the odds favor Carter's sticking to the 3 percent figure and that some of the uncertainty in Washington is a manuever to give the impression that the president is not ignoring pressure from other quarters.

Brown seemed to brush aside suggestions that the administration might seek some partial solution, such as a 3 percent increase only in that part of the U.S. defense budge related to NATO. The defense secretary pointed out that the original commitment was to an increase in defense spending meaning the total budget. "I don't think there is any doubt about what is meant," he said.

The concern whether the president would go back on a pledge he had iniated go back on a pledge he had initiated also was somewhat compounded here today by news that Secretary of State Cyrus Vance will skip the two-day NATO foreign ministers' meeting that opens here Thursday.

Vance will travel to the Middle East next week for another effort to bring Egypt and Israel together on a peace treaty. Although there is understanding here of the importance of that mission, it was not gone unnoticed that Vance will not yet be in the Middle East while the NATO foreign ministers are meeting and will stop in London on the way to the Middle East to give a speech9

Despite some distraction over events in Washington, the defense ministers' meeting here was described by Brown as "notable for successes," primarily because the Alliance signed a "memorandum of understanding" approving purchase of 18 U.S.-developed Airborne Warning and Control System (AWACS) planes. This $1.8 billion project has been a controversial focus of NATO debate for three years and its approval-with exception of Belgium, which does not have a permanent government at this point-"signals more effective cooperation and a more effective defense capability," Brown said.

The AWACS are modified Boeing 707 jets cranned with radar and computers. They are meant to look a few hundred miles into enemy territory with their electronic eyes and provide an additional 15 minutes or so of warning time an enemy air attack.

The planes, to be based in West Germany, would patrol the skies from Norway to Italy. The United States is paying 35 percent of the cost, West Germany 30 percent and the other countries the balance, officials said today. The Boeing fleet, which will begin operating in about three years, will be augmented by 11 British-built Nimrod control planes.

Brown met privately here last night with German Defense Minister Hans Apel and is said to have invited Apel to Washington in February. The West German defense chief has quickly become a controversial figure around the NATO table and is said to have asked for a long discussion with Brown on how to deal with the so-called "gray area," nuclear weapons of intermediate range that do not conveniently fall into U.S. arms negotiations with the Soviets.

Brown also talked privately with British Defense Minister Fred Mulley here and reportedly sought to discourage a possible British sale of Harrier jump-jets to China for fear it would upset the strategic arms talks with the Soviets.