The United States publicly and pointedly reaffirmed its 20-year-old commitment to the security of Pakistan yesterday, as its next door neighbor, Afghanistan, was reported invaded by new, heavily armed military units of the Soviet Union.

Presidential national security affairs adviser Zbigniew Brzezinski, speaking on "Issues and Answers" (ABC, WJLA), read portions of the 1959 U.S.-Pakistan agreement, as a public message to Pakistan and the world that "it is an important commitment and the United States will stand by it."

Brzezinski said he had been specifically authorized to reaffirm the U.S. commitment, presumably by President Carter.

There is no indication that the continuing flow of Soviet forces into Afghanistan, described by Brzezinski as "direct invasion" and "large-scale aggression," is likely to move on to Pakistan in the short run. However, a Soviet-dominated Afghanistan manned with Soviet combat forces would be an ominous new fact of life for already unstable Pakistan to deal with, and would present a long-range military threat of major dimensions.

The 1959 commitment read by Brzezinski calls for the United States, in case of communist aggression against Pakistan, to take "appropriate action, including the use of armed force, as agreed by the two nations and in accordance with U.S. constitutional procedures."

Official sources said the commitment has been reaffirmed privately on at least three previous occasions in the last year, as Pakistan worried about conditions in Afghanistan and about repeated charges and warnings from Moscow concerning alleged Pakistani aid to Moslem insurgents battling pro-Soviet governments in Afghanistan.

After a telephone conversation Friday between Carter and Gen. Mohammad Zia ul Haq, Pakistan's president, U.S. Ambassador Arthur Hummel began intensive discussions with Pakistani officials about new measures to assist that country and assure its security.

These discussions, which are reported here to be still in an early stage, are complicated by the application of U.S. nonproliferation laws barring economic and military assistance to countries seeking to produce or acquire nuclear weapons for the first time. Early this year the United States cut off aid to Pakistan, except for food assistance under these laws because of Pakistan's secret drive to build an atomic weapons capability.

Official sources said it is unlikely that Pakistan will stop its nuclear development. In this circumstances, direct U.S. help will be limited by law to food aid and cash sales of military equipment and supplies.

Carter, in a White House luncheon with reporters Saturday, made known his resolve to speed up delivery to Pakistan of purchased weapons and spare parts, estimated to be about $150 million worth of armored personnel carriers, tactical missiles, ammunition and spare parts.

Pakistan has made no new weapons request to Washington since the open Soviet moves in Afghanistan began a week ago, according to officials, not is it clear what role the Pakistanis envision for the United States in view of the still growing Soviet presence across the border.

Islamic fundamentalism and anti-American sentiments in Pakistan have been stirred by the strident appeals of Ayatollah Rubollah Khomeinini and other figures in Iran, another important Pakistani neighbor.On Nov. 21, a Pakistani mob attacked and burned the U.S. Embassy in Islambad, leading to the departure of many Americans.

New signs of Pakistan's sensitivity to its Iranian neighbor were official statements over the weekend that Pakistan opposes the use of U.S. force against Iran, where 50 American have been held hostage since Nov. 4, and that it takes exception to the U.S. freeze of Iranian financial assets. This suggests that despite an enhanced Soviet military threat, Pakistan may choose to continue a low profile relationship with Washington while the U.S.-Iranian conflict continues.

Brzezinski, in his television interview, said there is increasing evidence of large-scale Soviet troop movements into Afghanistan at two points along the border: from Soviet Kushka into the Afghan city of Herat and from Soviet Termez toward the Afghan capital of Kabul and the nearby airport at Bagram.

The national security adviser said the Soviet forces include "armored formations, a large number of heavy tanks, the most modern Soviet tanks, Soviet armored personnel carriers, motorized infantry and so forth." He said that Saturday's official estimate of 20,000 to 25,000 Soviet combat troops, plus about 5,000 other Soviet military personnel, probably has been exceeded, but officials did not release a higher total yesterday.

Of greater potential significance than the continuing movement across the Afghan border are intelligence reports that large numbers of additional combat units are being moved within the Soviet Union in ways that suggest they may be headed to the border area for assignment to the Afghan front.

The Soviet forces that have entered Afghanistan so far may be adequate to secure key cities, airports, and important roads, according to U.S. officials. Movement of additional units on a large scale to Afghanistan in days to come would be taken here as a sign that the Soviets intend all-out military operations against rebellous Islamic tribesmen.

Brzezinski declined to say whether the United States is considering military aid to the rebel forces in Afghanistan. However, informed sources indicated that this is among the subjects to be discussed in London today by Deputy Secretary of State Warren Christopher with senior officials of Britain, France, West Germany, Italy and Canada in a meeting on possible responses to the situation in Afghanistan.

Christopher, boarding a Concorde flight to London, called the Soviet intervention "a grave threat to international stability." He added, "I think the world community is so outraged that the Soviets will find in the long run that it will be most costly to them."

Following the London meeting, Christopher is scheduled to go to Brussels, where a special meeting of the North Atlantic Treaty Organization council has been scheduled Tuesday to discuss Afghanistan.