President Carter's national security adviser, Zbigniew Brzezinski, landed here tonight to personally assure Pakistan of America's seriousness in defending this country and the Persian Gulf against Soviet threats from Afghanistan.

At the same time, U.S. officials made it clear they are not on a negotiating mission to increase the $400 million they have promised Pakistan over the next two years -- an amount Pakistan's President Mohammed Zia-ul-Haq has called "peanuts" and has said will not meet his country's need, but only will provoke the Soviets.

Legislation to authorize a new economic and military aid program for Pakistan will be "open-ended" and thus will permit future U.S. assistance beyond the two-year, $400 million program currently proposed, State Department officials said yesterday in Washington.

[The officials said, however, that no decision had been made to offer specific amounts of aid to Pakistan beyond the two-year program.]

The Pakistanis have made it clear they hope the talks over the next two days here will mean increased U.S. aid. To entice the United States into offering more money, Pakistani sources and newspapers here cite reports of a $2.6 billion military aid package that the Soviets reputedly have signed with India. Indian officials here and in New Delhi deny any such deal, and Western intelligence sources said they know nothing about such a large arms pact.

Floating such a story at this time highlights the regional revalries that, with Pakistan's desire for a heavy commitment of aid from the United States, make Brzezinski's mission a difficult one.

"We are not coming here to be bargained up," said a U.S. official. "This is not a negotiating mission. This is a fact-finding and consultative mission."

Brezezinski was accompanied here by Deputy Secretary of State Warren Christopher and Assistant Secretary of Defense David E. McGiffert. A team of 15 Pentagon experts will arrive Saturday to go into the details of Pakistan's military needs.

The trip originally was to be made by Christopher, but was upgraded to include Brzezinski to emphasise Carter's State of the Union declaration of U.S. intent to defend Pakistan and the Persian Gulf against Soviet aggression.