The United States had decided to put off bringing its $400 million Pakistan aid package to Congress until it determines how much other nations will contribute, national adviser Zbigniew Brzezinski announced tonight after two days of talks here with Pakistani leaders.

In their talks, which lasted more than two days, the two sides also further defined their 1959 defense agreement, under which the United States would aid Pakistan in the event of a Soviet attack, would not come into play if the Soviets attacked in platoonor battalion strength. "But a genuine threat to the independence and integrity of Pakistan" would draw the United States in, an official said.

Both An American source in the delegation and Pakistani President Mohammed Zia ul-Haq put the best face on the talks, despite their apparent inability to come up with answers to the hard questions facing the two nations, which once were firm allies andnow are being drawn back together by the threat of Soviet forces in neighboring Afghanistan.

Brzezinski and Deputy Secretary of State Warren Christopher leave here Monday for Saudia Arabia, the Islamic state whose support they would like to enlist in helping Pakistan.

Christopher was scheduled to testify before Congress on the Pakistani aid program this week, soon after returning from the trip.

But Brzezinski said tonight that the Carter administration feels it will be in a better position to make a final determination on Pakistan's overall needs after consulting with other nations. In addition, Pakistan indicated that it wants to see the total international aid package before committing itself to the American offer.

"We want to look at a longer range basis," the American source said. A delegation source termed the $400 million "seed money for larger effort."

But Zia, who called the $400 million "seed money for a larger effort." clear tonight in an interview with five American reporters that the money issue no longer separates the United States and Pakistan.

He said combination of President Carter's strong affirmative pledge to come to the aid of Pakistan if it is threatened together with Brzezinski's trip here convinced him of America's full commitment to Pakistan.

Backing down from his Jan. 1 call for a full-fledged treaty, he said a Carter plan to seek congressional reaffirmation of the 1959 U.S.-Pakistan defense pact, an executive agreement, now is enough. He appeared in no rush to get that congressional reaffirmation, which was scheduled to be included with the $400 million aid request. Moreover the United States and Pakistan seemed at accord over what the 1959 agreement means -- an issue that caused wide splits when Pakistan wanted American help in its 1965 and 1971 wars with India.

Both Christopher and Zia said the agreement refers to aggression by the Soviets or a Soviet-dominated nation.

Zia said he does not expect the United States to guard Pakistan against small skirmishes with Soviet troops on its border. But if the Soviets mount a full-scale attack, he said he was confident America would move in.

A source in the American delegation said the United States wants Pakistan to have the ability to defend itself against Soviet border raids.

If the Soviets pull back, the source said, it would be all the better because Pakistan would have proved its military ability. But if the Soviets escalate, "They have us engaged," subject to the limitations of the agreement, which calls for consultation with Congress.

Despite the optimistic tone -- the delegation source said, "The vibes, the attitudes were much better than expected" -- the hard issues separating the two countries still remain.

Getting other nations to join a substantial aid program could be a problem, the American delegation source said, and Pakistan's desire for massive amounts of military hardware may have to be toned down.

Beyond that, the United States still is concerned about Pakistan's nuclear program, which it believes is headed toward atomic weapons development, despite strong denials.

Nonetheless, the American delegation source said, the new understandings reached between the United States and Pakistani leaders will enable the two nations to better tackle those issues.

The most important new elements leading to the understanding are the "shared perceptions" of the Soviet threat and Pakistan's belief that the United States is a reliable long-term ally.

When Brzezinski and Christopher go to Saudi Arabia Monday morning, they will leave behind here a military team to assess Pakistan's needs. Zia said that team should do its job within four days.

The American source said the $400 million aid figure for the next 18 months -- equally divided between military and economic help -- was arrived at in a rush to bolster Pakistan's confidence that the United States sat firmly in its corner.

As a result of the last two days of talks, the source said, it became clear that Pakistan no longer needs those quick assurances and both sides can now take their time to come up with a more detailed, longer range aid package. The source made it clear, however, that the original $400 million is unlikely to be increased.

Zia said the greatest value of the talks "is the United States of America's interest in this region, about which in the past I have been a little doubtful."

It was clear from the interview with Zia that the Pakisani president was both impressed with Brzezinski and flattered that Carter's top national security adviser came here to talk with him. He called Brzezinski "a very able guy."