The Reagan administration, not convinced that the Soviet Union is ready to withdraw its troops from Afghanistan, has decided to send the U.S.-armed Afghan insurgency long-range mortars and mine-clearing equipment to help the guerrillas attack Soviet and Afghan military bases.

The decision to send heavy 120-mm. mortars and explosive cords to clear a path through minefields was made earlier this summer, before the failure of the latest round of peace talks in Geneva between Pakistan and the communist Kabul government, according to administration and other sources.

Rep. Charles Wilson (D-Tex.), a member of the House Permanent Select Committee on Intelligence and a strong supporter of the Afghan resistance, confirmed the reports and predicted that the mortars and special mine-clearing equipment would arrive in Afghanistan "by the time snow falls" there, normally in October or November.

The decision reflects a general U.S. government assessment that the Soviets, while anxious to extricate their estimated 115,000 troops from Afghanistan, still have not made the hard decision to do so if it risks the defeat of the weak Afghan government by the anti-communist rebels.

The administration earlier this year decided to step up pressure on the Soviets by sending hundreds of Stinger antiaircraft missiles to the insurgents, who have proven highly effective in using the weapon against Soviet aircraft.

Soviet officials have complained bitterly about this escalation in U.S.-provided military technology and said that this is only making it more difficult for them to get out. They are likely to view the latest U.S. decision as further evidence that the administration is more interested in a Soviet defeat in Afghanistan than a political settlement.

The Afghan insurgents have been pressing for the mortars and mine-clearing equipment so that they can begin laying siege more effectively to the eight major Soviet air bases and roughly 30 smaller Soviet or Afghan garrisons with airstrips scattered around Afghanistan. The mortars, with a longer range than those currently in insurgent hands, would allow attacks from a greater distance, and the mine-clearing equipment would allow the insurgents to penetrate isolated bases.

U.S. intelligence officials estimate that no more than 15,000 or so of the 115,000 Soviet troops in Afghanistan are actively engaged in combat in the field. The remaining 100,000 or so are involved in logistic operations and as security forces at airfields and in urban areas. The 120-mm. mortars would increase the ability of the guerrillas to harass rear-area troops.

Still to be decided, according to informed sources, is whether the administration will send a Spanish- or Israeli-made version of the 120-mm. mortar, both of which are far lighter than the comparable 107-mm. U.S. mortar.

The Spanish and Israeli light mortars have a range of roughly 3.8 miles. But the Spanish model is much lighter and thus more adaptable to the guerrilla warfare in mountainous Afghanistan, where mules are the main means of transportation.

The new U.S.-provided arms are unlikely to have any effect on the battlefield until next summer, however, because winter conditions make heavy fighting extremely difficult.

Wilson predicted that if the insurgents could get through the minefields that surround Soviet and Afghan air bases and garrisons, a larger number of government soldiers would defect. The minefields, according to Wilson and resistance sources, have been a major obstacle preventing many Afghan army soldiers from getting away from isolated garrisons.

Wilson, who has visited rebel-held territory inside Afghanistan, also predicted that if the insurgents have the same success next summer in fighting Soviet elite troops as they did this summer, they would force a Soviet withdrawal by the end of next year.

In the latest round of the Geneva negotiations early this month, the Soviets had been expected to cut the timetable for their withdrawal from 18 months to one year. Instead, the Afghan delegation offered only a two-month reduction in the Soviet withdrawal timetable.

Pakistani and U.S. officials have concluded that Kabul government leaders, worried that they could not survive such an abrupt Soviet withdrawal, persuaded Moscow to hold off on the anticipated one-year offer.

The continuing escalation in U.S. military aid to the Afghan rebels and the failure in the latest round of Geneva talks provide a sharp reminder that the United States and Soviet Union -- unlike their arms control negotiations -- are still far from reaching any agreement on the conflict in Afghanistan.

U.S. officials report no significant progress or even a hint of a Soviet decision to withdraw finally in the latest series of U.S.-Soviet exchanges on regional conflicts.

Secretary of State George P. Shultz said Friday that he and his Soviet counterpart, Foreign Minister Eduard Shevardnadze, had "the most thorough and searching discussion" they have ever had on the Afghan issue. But he said "there wasn't any movement on a time schedule."