The Reagan administration sought yesterday to play down the significance of Babrak Karmal's sudden resignation as president of Afghanistan, saying it was of less importance than the continuing presence of Soviet troops there.

Officials also raised the possibility that the resignation was connected to the latest round of U.N.-sponsored peace talks opening today in Geneva.

"We would not attach undue importance to the changeover in leadership in Kabul," said State Department spokesman Anita Stockman, who called Babrak's replacement, Najibullah, the Afghan secret police boss, a close ally of Babrak.

"The identities of those who hold leadership positions are of less significance than the continued presence of 120,000 Soviet troops in Afghanistan," she said.

Stockman said the replacement of Babrak may be connected with the new round of "proximity talks" in Geneva between Pakistan and the Soviet-installed Afghan government. "The important issue is what hard proposals regarding the withdrawal of the Soviet forces from Afghanistan are put forward in Geneva," she said.

Last week, a senior administration official predicted the Soviets might be preparing "to dump" Babrak to gain "a fig leaf of legitimacy" for the Soviet-backed government in Afghanistan. Babrak had just returned from Moscow after a long absence officially attributed to his need for medical treatment.

The official, Deputy Assistant Secretary of State Robert A. Peck, described the two men reportedly then being considered as his replacement, Prime Minister Soltan Ali Keshtmand and Najibullah, as "equally obsequious" to Moscow.

Even before yesterday's news of Babrak's political demise, Reagan administration officials were dismissing recent diplomatic maneuvers by the Soviet Union as "largely tactical" and saying the U.S. assessment had not changed that the real Soviet intent remained "the total subjugation of the Afghan people."

Reports that the Soviets have for the first time drawn up a timetable for troop withdrawal over a 12-month period, plus recent conciliatory comments by Soviet leader Mikhail Gorbachev, have raised expectations of a possible breakthrough in the four-year-long negotiations at this round of the U.N.-sponsored talks.

These expectations have in turn hightened concern among Afghan anti-communist guerrilla groups and their U.S. supporters in and out of Congress that the administration may be ready to end its support for the resistance as part of the package deal being negotiated by U.N. Undersecretary Diego Cordovez. None of the guerrilla groups is participating in the talks.

In March, however, the administration decided to escalate its support for the Afghan guerrillas by sending them shoulder-fired, antiaircraft Stinger missiles to defend themselves against Soviet Hind helicopter gunships.

In testimony Thursday before the House Foreign Affairs subcommittee on Asia, Peck sought to reassure the Afghan guerrillas and their congressional backers that Washington was not "on the verge of cutting a deal [with the Soviets] behind the backs of the Afghan people."

Under sharp questioning from Sen. Gordon J. Humphrey (R-N.H.), appearing as a witness, Peck said the administration had insisted that the year-old Resistance Alliance, a coalition of Afghan guerrilla groups, be "consulted" about the negotiations. But he indicated it was too late to bring the alliance directly into the U.N.-sponsored negotiations.

He also said the United States would not act as "a guarantor" of any agreement -- an offer made last December that angered American conservatives and sent jitters through the Afghan resistance -- until it was satisfied the accord would bring about "the prompt withdrawal" of Soviet troops.

Peck said, however, that the Soviets had recently increased the size of their combat force in Afghanistan, become more directly involved in the war against the U.S.-supported Afghan resistance and brought in more sophisticated arms to crush it.

"Moscow is continuing to pursue vigorously a multi-faceted strategy apparently aimed at achieving the total subjugation of the Afghan people," Peck said in his prepared testimony.

"Soviet actions on the ground in Afghanistan reinforce our assessment that their diplomatic efforts are largely tactical, designed to strengthen their military and political position in Afghanistan and in the eyes of world opnion while they continue their efforts to subjugate the Afghans," he added later.

The Soviet two-track policy of escalating its military effort on the ground while suggesting a new flexibility at the negotiating table has nonetheless led some U.S. officials to express concern that Gorbachev may succeed in undermining Pakistan's support for the Afghan resistance.

They fear the new Soviet diplomacy could tempt the Pakistanis into recognition of, and direct talks with, the Afghan government. To date, Pakistan has refused both, requiring U.N. mediator Cordovez to conduct indirect talks between the two parties.