President Reagan assured Afghan resistance leaders yesterday of increased American military and political support to help them bring about a Soviet withdrawal from Afghanistan but also signaled Moscow he is ready to help it achieve what he called "a genuine political settlement."

"The support that the United States has been providing the resistance will be strengthened, rather than diminished, so that it can continue to fight effectively for freedom," Reagan said after a 20-minute meeting at the White House with Maulavi Yunis Khalis, chairman of the Islamic Alliance of Afghan Mujaheddin (holy fighters).

Four other Afghan leaders from the seven parties in the U.S.-backed alliance were also present at what Reagan called "a very moving discussion" of the bloody eight-year-old war.

Reagan gave no hint of what additional U.S. military assistance he had in mind. But a congressional source familiar with the CIA-run covert military program, which now costs more than $500 million annually, said it was more "a steady, planned, methodical increase to sustain what's there" rather than "a dramatic increase."

Khalis, the newly elected first president of the alliance, said he had no doubt that Reagan would keep his promise of continued support for the resistance in a war that he predicted is likely to continue.

The rebel leader said the alliance would continue its struggle until the Soviets left and a government "that the Afghans accept comes to power. We do not accept the preferences of others as far as our government is concerned."

Afghan resistance leaders are known to worry that Reagan and Soviet leader Mikhail Gorbachev might reach a compromise settlement of the Afghan conflict at their summit here next month.

They are concerned that a compromise might leave them out or force them to enter a coalition dominated by the Soviet-backed Afghan Communist Party.

Reagan's comments yesterday seemed aimed at reassuring the alliance that the United States does not intend to "sell out" the Afghan resistance.

However, administration officials say alliance leaders must begin thinking about some kind of "interim government" in which they will have to share power with the present Kabul government, at least initially, to provide the Soviet Union with a face-saving withdrawal formula.

Such a formula is expected to be a main point of the Afghan alliance leaders' discussions here with administration officials. They met yesterday with Undersecretary of State Michael H. Armacost and are scheduled to see Secretary of State George P. Shultz today.

Visiting with Khalis, who leads the Hisb-i-Islami Afghanistan, is Sibghattallah Mojadidi, leader of the Afghan National Liberation Front; Ahmed Gailani, head of the National Liberation Front of Afghanistan, and the deputy chiefs of three other groups in the U.S.-backed alliance.

In his comments yesterday, Reagan said "the key" to resolving the Afghan conflict remains a rapid withdrawal of the 115,000 Soviet troops stationed in Afghanistan.

He attacked the Soviet Union for announcing early this year "a phantom cease-fire" in the war and for its continuing "silence" on naming a firm date for a rapid withdrawal of its troops, despite Gorbachev's statements that Soviet forces are ready to leave.

Reagan also said the Soviet proposal for a transitional coalition government dominated by the Afghan Communist Party was unacceptable because it would leave "this discredited and doomed group in control."

But Reagan did not speak of extending formal diplomatic recognition to the alliance as Khalis called for in his comments.

The president also said that once the Soviet Union showed convincingly that it was ready for a genuine political settlement, the United States would be helpful.

Administration officials have said previously that they would regard a Soviet offer of a one-year timetable for withdrawal as a major indication of Soviet seriousness. Moscow's latest offer, made at U.N.-sponsored Afghan peace talks in Geneva in September, was to withdraw its forces over a 16-month period.