The Soviet Union, despite a stiff message from President Carter via the Washington-Moscow "hot line," was reported yesterday to have sent two motorized infantry divisions rolling into Afghanistan, tripling the size of the Soviet combat force reported in that country.

White House press secretary Jody Powell called the large-scale movements "a Soviet invasion" and placed the total number of Soviet combat troops in Afghanistan at 25,000 to 30,000. The equivalent of one airborne infantry division has been deployed separately near the capital city of Kabul as the result of a Soviet airlift that started at dawn last Monday. o

There was no immediate explanation for the rapid expansion of the Soviet military presence. U.S. officials noted that even the augmented force would be far from enough for a nationwide campaign against armed Islamic tribesmen in Afghanistan. There was speculation that the new force might hold cities and lines of communication while what is left of the Afghan Army fights the rebels, or that it might be the advance element of an even larger Soviet combat presence to fight the rebels.

This tangible military response by the Soviets to U.S. and worldwide protests was reported to Carter yesterday morning before a written message from Soviet President Leonid I. Brezhnev began clattering across the Pentagon terminal of the teletype "hot line" around noon.

White House officials declined to describe the tone or substance of Brezhnev's message. But sources made it clear that Moscow turned a cold shoulder to Carter's appeal, dispatched Friday, for a Soviet withdrawal from Afghanistan.

According to an authoritative account, Carter informed Brezhnev that continuation of the Soviet drive in Afghanistan would have serious adverse consequences for U.S.-Soviet relations. Carter reportedly charged that the Soviet action, which he publicly condemned Friday as "a grave threat to peace," is a violation of the 1972 agreement on "basic principles of U.S.-Soviet relations" in which the two nations pledged to avoid military confrontations, exercise restraint and seek peaceful resolutions of disputes.

The Soviet military action in Afghanistan and overthrow of the previous Afghan leadership has introduced a new element into Carter's thinking about the 'iranian crisis as well as Middle East-South Asian regional problems and U.S.-Soviet relations.Carter's thinking was made known at a White House luncheon yesterday with 12 Washington journalists, on condition that the authoritative source of the views and information not be quoted directly.

Among the highlights of the luncheon table disclosures were:

Carter's determination to continue to increase the pressures on Iran in a steady, measured fashion, with an international consensus behind him, to bring about release of the U.S. hostages who have been held in the U.S. Embassy in Tehran for eight weeks today.

The United Nations sanctions that Washington has requested, and which Carter appears confident of obtaining, are seen as a major additional step in this effort. So are economic measures likely to be imposed by major U.S. European allies and Japan, in keeping with a consensus among them which Carter believes has been reached.

The interruption of commerce to Iran is not considered to be the limit of the internationally approved measure that could be adopted should Iran continue to defy United Nations Security Council resolutions and an International Court of Justice decision. Carter is conscious that Section 42 of Chapter vii of the United Nations charter authorizes blockades and other military action by land, sea or air forces of U.N. members to enforce Security Council resolutions. There was at least a suggestion that the United States might ask for U.N. authorization under this part of the charter if military action against Irean were deemed necessary.

Carter's belief that the Soviet military moves in Afghanistan, Iran's neighbor to the east, will be a sobering new element in the calculations of Iran's leadership. In his opinion, it is now clear that Iran faces a very serious threat from the Soviets, who have not always been friendly neighbors.

The unstated implication is that Iranian relations with the United States, a traditional counterweight to a Soviet threat, now become more important in Tehran.

Carter's uncertainty, bordering on puzzlement, about the reasons for the Soviet thrust into Afghanistan in recent days. Unlike his national security affairs adviser, Zbigniew Brzezinski, who regards the move as consistent with the Soviet pattern, Carter considers it a departure from the Soviet posture since the end of World War ii.

One possible motive behind the drive, in his view, is the historical Soviet quest for access to warm-water outlets to the Indian Ocean, but the fulfillment of this aim would require control or acquisition of parts of Iran or Pakistan lying between Afghanistan and the sea.

To protect Pakistan, which Carter hopes and expects will retain its independence, intensive discussions have been started in Islamabad between Gen. Mohammed Zia ul Haq, Pakistan's president, other high Pakistani officials and U.S. Ambassador Arthur Hummel.

The United States terminated economic and military aid to Pakistan early this year because of Washington's belief that the Pakistanis are engaged in building an atomic bomb capability in a secret plant, outside of international atomic energy safeguards. There appears to be little likelihood, according to other U.S. officials, that Pakistan will agree to halt its nuclear program in a way that makes possible the resumption of aid under existing U.S. laws aimed at nuclear proliferation.

However, the U.S. laws permit cash sales of military weapons and supplies to Pakistan in present conditions, and Carter believes that decisions on and deliveries of items which Pakistan needs should be expedited.

About 300,000 refugees from Afghanistan are reported to be encamped on the Pakistani side of the porous, virtually unpatrolled mountainous border of those two nations. The Soviet Union and the successive communist regimes in Afghanistan since early last year have charged that the tribal insurgency in Afghanistan is armed and aided through these camps with Pakistani authorization. Pakistan has denied the charges.

Carter appears certain that the deeply religious, highly motivated Afghan tribesmen will continue to present severe problems for a Soviet-dominated regime in Afghanistan, but he is unwilling to discuss the possibility of U.S. aid or U.S. recognition of some sort for these insurgents. There was no sign at yesterday's lunch that he has ruled this out.

Carter's belief that a range of unspecified political and economic options is available to the United States in response to the Soviet military action in Afghanistan. His discussions with European leaders and communications in the past two days with 20 to 25 leaders of "Third World" countries convinces him of the strong consensus against the Soviet action. He hopes that other nations will speak out against the Soviet threat to the peace on their own, without U.S. orchestration.

Carter's nearly fixed decisions about the planned trip to Peking starting late next week by Secretary of Defense Harold Brown, the first high-level military contact since normalization of U.S.-Chinese relations. Carter expects it to be primarily for consultation and exploration.

The president continues to reject U.S. military sales to China or "playing the China card" to bedevil the Soviet Union, but he believes that the United States and China have common strategic goals and is gratified that China has not disappointed the United States in any instance since the establishment of full relations Jan. 1, 1979.