Carter administration officials yesterday revised their estimates of the Soviet troop presence in Afghanistan, putting the total at 70,000 in contrast to the 90,000 to 100,000 figures issued by the State Department during recent weeks.

An administration official said last night that the downward revision represents a refined estimate by the government's intelligence community, not a Soviet withdrawal. Another 30,000 Soviet troops are on the Afghan border, he said.

The higher estimate, he added, stemmed from counting elements of a division in Afghanistan as a full division.

The purpose of issuing the refined estimate last night, administration officials acknowledged, was to undercut any Soviet claims that they had begun their withdrawal from Afghanistan. President Carter demanded, as a condition to U.S. participation in the Moscow Olympics, that the Soviets withdraw all their troops by Feb. 20.

Meanwhile, U.S. officials announced that a second flight of B52s has been sent over the Arabian Sea and a third will be launched soon. The second flight of two bombers, like the first, flew from Guam and returned there.

The purpose is to impress the Soviets with the long range of U.S. air-power in hopes of deterring them from taking offensive action in the Persian Gulf. The first flight was Jan. 20 and the second Feb. 5. A third is expected within two weeks.