A leading defense specialist in West Germany's opposition party has threatened to block Bonn's participation in $1.8 billion NATO project to buy U.S.-built earlywarning radar planes unless the United States speeds up its promised compensatory purchases of German products.

The threat came in a letter from Christian Democrat Carl Damm to Bonn Defense Minister Hans Apel. Damm called on Apel to warn U.S. Secretary of Defense Harold Brown of the seriousness of the situation, and to urge that the United States "fulfill its responsibilities."

Damm is an influential member of the defense committee of parliament, and it is generally felt here that he has enough support to carry out his threat.

The letter reflects growing impatience in some quarters over getting the united States to live up to its commitment to a "two-way street" in international military hardware purchases. This frustration arises from a perception that the U.S. military, Congress or industrial lobbies can show or derail foreign purchases to which the U.S. administration has agreed.

"as a parliament, we have to deal with the United States of America as a whole," said Damm in an interview, and not with the U.S. Armey, trucking lobbies or the Congress.

After years of cobtroversy and debate, NATO last year agreed to buy 18 of the big Boeing radar planes -- KNOWN AS AWACS, for airborne warning and control system -- with West Germany picking up 30 percent of the cost, the United States paying 42 percent, and the other NATO nations sharing the remainder. In return for its major support, however, bonn was promised a number of compensatory deals, including the U.S. military purchase of some 9,000 German vehicles worth $111 million and a german telephone system to replace the antiquated U.S. military network, for about $105 million.

Damm complained that rather than some 1,500 vehicles a year, the United States thus far has purchased only 282 vehicles and has not let the contract for the phone system.

In Damm's view, that is not nearly enough to give parliamentary committees here confidence that the united States will live up to its pledge, given its historical lack of interest in foreign purchases.

"I'm in favor of AWACS and don't want to kill it," Damn said. "What I'm trying to do is send a warning signal to the U.S. government -- three or four months before the defense committee here meets -- that if nothing happen before them, it is my feeling that we will not release the next $55 million in our 1980 budget for AWACS."

Another cause of German frustration, Damm said, is that purchase of the French-German Roland air defense missile is still stalled in congressional disagreement and the U.S. Army, rather than buying 10-ton trucks already in service with the German army, is going out for bids on another, similar vehicle.

Andreas von Buelow, the number two man in Bonn's Defense Ministry, said in a radio interview, "If we should come to the conclusion that the American attitude is developing along the lines suspected by Damm, we would consider this a basis for destroying the contract." He added, "But for the time being this is not so."

Damm acknowledged that he has no hard evidence that the United States will eventually fulfill these projects. U.S. officials in Europe admit the truck program is going more slowly than it should, but they also say there is no intention to back out.