Defense ministers of the North Atlantic Treaty Organization agreed today on a six-year, $7.85 billion package of basic improvements in conventional defense to upgrade ground equipment and boost ammunition stocks in response to U.S. criticism of the alliance's infrastructure.

The accord calls for NATO to build or improve aircraft shelters, fuel pipelines, communications and munitions dumps to enhance the ability of the United States to reinforce Western Europe during a military crisis.

The 14 ministers, representing all NATO members except France and Iceland, also affirmed that each country will strive to attain 30 days' worth of ammunition stocks for all key weapons to strengthen capacity to fight prolonged wars without resorting to early use of nuclear weapons.

Some NATO countries are said to possess ammunition stocks that would last only a week to 10 days if war broke out.

Richard Perle, assistant secretary of defense for international security, hailed the session as "the most satisfying and most successful" that NATO defense ministers have held in recent years. He noted that the new allocation represents more than double the amount NATO countries invested in infrastructure and ammunition during the previous six-year installment.

The plans to improve "the nuts and bolts" of the alliance's conventional forces have gained new urgency in recent months because of the proposal last June by Sen. Sam Nunn (D-Ga.) to withdraw up to one-third of the 326,000 American troops in Europe unless the allies took steps to bolster their own nonnuclear defenses. Nunn's initiative was narrowly defeated, but fellow senators hinted that similar proposals may come before the new congressional session.

Perle sought to deflate the threat of such legislation by contending that the program to upgrade infrastructure and ammunition supplies had been under consideration for more than two years and that NATO would have agreed on the plan even "without Sen. Nunn's concurrence."

Perle said he was impressed by the cooperative tone of the European allies at the meeting and the fact that they "did not have to be dragged kicking and screaming" to approve the program.

But European officials indicated that the impact of Nunn's initiative was important because he is widely regarded as a firm supporter of the NATO alliance. They said there was a greater determination now in Europe to underscore the significance of Europe's involvement in allied defenses but that this by no means reflected a willingness to provide a "blank check" on expensive investments in sophisticated new weaponry.

"We are very determined that the efforts we are making should be recognized as the vital contribution that they are," said British Defense Minister Michael Heseltine, who chaired a separate meeting of European ministers before the full NATO session.

Heseltine noted that the "Eurogroup" countries had managed sustained increases of 2 percent a year in defense spending over the past decade. NATO's spending goal now, adopted by the defense ministers in 1977, is a 3 percent hike each year after inflation.

Some NATO defense planning specialists have argued that the 3 percent goal is increasingly unrealistic at a time of prolonged unemployment and low growth among the European economies. They contend that it is far more important to assess what is being done on the ground in terms of improved defense capabilities.

The new infrastructure plan is one of the cheaper initiatives undertaken by the alliance in recent years. But it is also considered to be crucial if the alliance wants to maintain a strong conventional bridge between the United States and European countries.

One of the premises behind Nunn's proposal is that the current American military buildup would be wasted in a military crisis in Europe. For instance, U.S. planes bringing reinforcements could be highly vulnerable to enemy attack because of allied defense limitations.

Moreover, the weakness of some European countries that lack adequate ammunition stocks could force U.S. troops to surrender early in a conflict because of insufficient support from allied troops.

Senior NATO officials said that today's program will pay for the construction of hundreds of new air shelters so that Europe would be able to shield 70 percent of the 1,500 tactical American aircraft designated to come to Europe's aid in wartime.

Planned improvements besides air defense include communications systems, sensory detectors to spot enemy forces, fuel pipelines and equipment storage depots. Investments in these areas will help to overcome what NATO military authorites have identified as "critical deficiencies" in the alliance's conventional force structure.

They said that such gaps have arisen over the years because many countries opted to spend their money on costly weapons systems without enough supplementary support and ammunition.

One of the most serious problems afflicting allied military forces that the new program is supposed to rectify is the lack of 155-mm artillery shells. The ministers asked NATO military commanders today to specify other "battle-decisive" munitions that are now in short supply so that each country can upgrade stocks.

[Warsaw Pact foreign ministers, meeting in East Berlin, called for serious negotiations with the West to reduce nuclear arms but said such talks should have clearly defined goals, according to a communique issued by the official East German news agency and quoted by Reuter. The communique said the "chance of a change for the better in the international situation now exists."]