The United States is sending a high-level diplomatic and military mission to Pakistan later this week to demonstrate support for that endangered country and to work out details of a proposed $400 million U.S. aid program, according to informed sources.

The mission, to be headed by presidential National Security Affairs Adviser Zbigniew Brzezinski and Deputy Secretary of State Warren Christopher, is scheduled to leave for Islamabad Thursday. The high officials may also visit Saudi Arabia before returning to Washington early next week.

A military delegation headed by Assistant Secretary of Defense David E.

McGiffert also will to to Pakistan late this week and is likely to remain there for technical discussions after the senior political officials depart, according to the sources.

Pakistan's president Gen. Mohammed Zia Ul-Haq, was bluntly criticized the U.S. aid offer, which was tendered to the top Pakistani foreign affairs official, Agha Shahi, in Washington Jan. 12. Zia publicly referred to the $400 million proposal as "peanuts" and "terribly disappointing." In addition to seeking more money, Zia asked that the United States convert its 1959 defense agreement into a formal "friendship treaty."

The Carter administration was upset by Zia's remarks but decided to swallow its indignation in the interests of international security and geopolitics. There is no sign that the United States will agree to increase its economic and military aid offer, however, and the administration is reported to be determined to avoid the protracted negotiation and ratification process that would be necessary to convert the 1959 executive agreement to a treaty.

Top U.S. officials, including President Carter, have repeated publicly the 1959 commitment to take action, consistent with U.S. constitutional processes in case of outside agression against Pakistan.

Carter said in his State of the Union address last Thursday that he will ask Congress specifically to reaffirm this commitment. The sources said this reaffirmation is likely to be included in the bill authorizing the aid package to Pakistan.

Because of sensitivity in Pakistan, official spokesman refused to confirm reports of the Brzezinski-Christopher mission yesterday. There were suggestions that an announcement of the trip is planned for tomorrow or Thursday, after the finish of the current Islamic foreign ministers' conference now meeting in Islamabad.

U.S. economic and military aid to Pakistan represents and abrupt turnabout for Washington, which cut off such aid in April 1979, because of evidence that Pakistan is building a secret uranium enrichment plant capable of making atomic bomb material. This aid cutoff was required under U.S. laws aimed at preventing the spread of nuclear weapons.

A special exception from the anti-proliferation laws will be necessary to carry out the proposed $400 million program, which is to span two fiscal years. c

The aid program for Pakistan and newly stated commitments to protect the Persian Gulf against outside threats are results of Washington's concern and policymaking following the Soviet invasion of Afghanistan.