WASHINGTON--President Reagan yesterday directed Secretary of State George P. Shultz to go to Geneva to sign "historic accords" by which the United States and Soviet Union will guarantee a peace agreement dictating the removal of all Red Army troops from Afghanistan.

Reagan called the pact a "triumph" for the U.S.-backed insurgents after a bloody, eight-year war with the Soviet-supported Kabul regime. He said the rebels "can count on our continued support."

The rebels have rejected the peace pact and have vowed to continue fighting. Under a compromise with Moscow, the United States will continue sending weapons to the rebels, while the Kremlin continues providing arms to the Marxist government.

Reagan made the announcement in the sun-splashed rose Garden before a ceremony to congratulate the mens' and womens' NCAA basketball champions.

"This is the first time in the history of the Soviet Union that they have moved by aggression into another country and then had to withdraw," Shultz said later at a White House press briefing.

The peace settlement, negotiated between Pakistan and Afghanistan under the auspices of the United Nations, is to be signed on Thursday.

Shultz acknowledged that the removal of Soviet troops will not necessarily mean an end to the fighting. Once the Red Army is withdrawn, Shultz said, "then the people of Afghanistan have got to work things out. That's their right and their problem...Perhaps we can get to a period of at least relative stability."

Beginning May 15, the Soviet Union is to begin withdrawing its 115,000 troops, with 50 percent of the forces removed over a three month period. All remaining Soviet forces are to be out by Feb. 15, 1989.

Shultz said Soviet Foreign Minister Eduard Shevardnadze has promised to complete the withdrawal by the end of the year, ahead of the deadline. He said the United States will "push for that and expect that."

While hailing the agreement, Shultz said, "the withdrawal of Soviet troops is obviously not the end of the matter." Negotiations will continue for the establishment of an interim government. "We know it's not easy. It will be hard," Shultz said.

He said the United States will help the rebels "as needed."

But on Capitol Hill, Sen. Gordon Humphrey (R.-N.H.) denounced the agreement as amounting to a "slow-motion sell-out" of the resistance fighters.

"It's a poor deal, a very bad deal," he said. "The Soviets are perfectly free under this agreement to resupply the communist puppet regime in Kabul just as lavishly as they please," while denying the United States the right to resupply the Afghan rebels.

The accord calls for the return to Afghanistan of millions of Afghan refugees, most of whom moved to neighboring Pakistan.