Reagan, announcing U.S. approval of the accord in the Rose Garden of the White House, stressed that he would not abandon the rebels.
"We take great pride in having assisted the Afghan people in this triumph, and they can count on our continued support," he said.
Both the Soviets and the Reagan administration have said they intend to continue providing military aid to their respective sides even as a U.N. mediator seeks to negotiate the establishment of an interim government to end the fighting.
Some conservative congressional critics of the President have accused the administration of selling out the rebel forces, but Reagan heatedly denied that charge.
Shultz, describing the terms of the accord at a news conference, said yesterday that the Soviets would begin withdrawing their 115,000 troops from Afghanistan on May 15, with half to be removed within three months.
The withdrawal is to be completed by Feb. 15, but Shultz said Soviet Foreign Minister Eduard A. Shevardnadze has told him he expects to have all troops out by the end of the year.
Shultz said the United States wanted the accord to include a joint U.S.-Soviet statement "that we each have the right to supply our friends in Afghanistan, but that we would have a moratorium on further supplies for a specified period of time."
But the Soviets refused to agree to any suspension of aid, so Shultz said ''we assert confidently our right to supply our friends in Afghanistan as we see the need to do so."
"And our sense of the need will be affected by whatever restraint we see on the part of the Soviets," Shultz added.
According to administration estimates, the United States now sends more than $600 million a year in military aid to Afghan rebel groups, which have vowed to continue fighting for the overthrow of the Kabul government rather than join in talks for a national reconciliation.
Shultz said the administration will support efforts by U.N. mediator Diego Cordovez to negotiate an interim government that will bring stability to Afghanistan so that about three million Afghan refugees who fled to neighboring Pakistan during the Soviet occupation can return home.
The two main parties to the Geneva agreement are Afghanistan and Pakistan, which has been anxious to reach an accord that would allow it to send the refugees back and end fighting along its border.