The Reagan administration yesterday offered a positive assessment of Soviet leader Mikhail Gorbachev's Monday announcement that he is ready to begin withdrawing Soviet troops from Afghanistan May 15, but pressed him to complete the process by the end of this year rather than by mid-March 1989.

White House spokesman Marlin Fitzwater said Gorbachev's declaration "does seem to take a very good step in the right direction." The State Department termed it "a positive signal of serious Soviet intent to withdraw from Afghanistan."

But State Department spokesman Charles E. Redman said the United States believes it is in the Soviets' interest to complete the withdrawal quickly once it begins. "We believe that could be done by the end of 1988 or even sooner," he said.

Under the Gorbachev proposal, the Soviets begin the withdrawal in mid-May and complete it by mid-March 1989, provided an overall peace agreement is reached by the middle of next month.

Fitzwater said the administration sticks to its position that a pullout should be completed by the end of this year, but realizes there are "a lot of contingencies" that might affect the timetable.

The growing seriousness with which the administration is taking the Gorbachev announcement was reflected yesterday in Undersecretary of State Michael H. Armacost's cancellation of a trip to Israel and the Persian Gulf Arab states to prepare for Secretary of State George P. Shultz's forthcoming trip to Moscow.

Armacost is the department's top specialist on Afghanistan. Redman said, "Afghanistan would seem to be an issue that would figure more highly this time" in Shultz's talks with Soviet Foreign Minister Eduard Shevardnadze.

Administration officials said yesterday they still had not been given the details of Gorbachev's plan. But Fitzwater said the administration was "pleased" by Gorbachev's statement indicating a large proportion of the 115,000 Soviet troops would leave early on in the process -- a key U.S. demand.

The White House spokesman sidestepped the issue of when U.S. aid to the Afghan resistance would cease, saying it still must be worked out. But apparently seeking to reassure the Soviets, he said "there is no question we're committed to stopping our support if there is a withdrawal."

"It's not really a very crucial issue as long as both sides agree on a proper pace of withdrawal and ceasing of support," Fitzwater added.

Administration spokesmen also left unclear whether the United States would sign the U.N.-sponsored peace accords being negotiated in Geneva if an interim government has not been established in Afghanistan.

Gorbachev said Monday the Soviets no longer insist agreement must be reached on an interim government before they begin withdrawal. But Pakistani President Mohammed Zia ul-Haq recently said Pakistan will not sign any peace accord with the present communist-led government in Kabul, insisting on the formation of a new one that is not dominated by communists.

Redman said the United States feels an interim Kabul government could "facilitate the withdrawal and the subsequent exercise of self-determination by the Afghan people" and that this is "an important subject" under discussion. But he gave no indication whether the administration will insist on the formation of a new government before signing an accord.

Meanwhile, Rep. Charles Wilson (D-Tex.), a foremost congressional supporter of the Afghan resistance, said one "crucial issue" that still must be resolved before any peace accord is likely to win congressional support is an end to Soviet military aid to the Kabul government in return for cessation of U.S. support for the Afghan rebels.