President Reagan's call for direct negotiations between the Soviet Union and Afghan insurgents is a welcome change in U.S. policy, but Afghan resistance groups have little interest in participating, a representative of seven major resistance groups said.

"It is useless for us to sit down with the Russians," said Gulbudin Hekmatyar, leader of the Hezb-i-Islami, one of the largest groups, in an interview yesterday. "For what? There is nothing to be discussed. They have invaded our country. The only solution is for them to withdraw their troops."

The resistance leader, however, applauded what he said was "a change in the policy of the United States: that the talks should take into account the views of the real parties," the Soviets and the Afghan resistance fighters.

The U.N.-sponsored talks in Geneva, involving the Soviet-backed Afghan government of Babrak Karmal and the Pakistani government "have not reflected the views of the mujaheddin," the Islamic guerrilla fighters who have fought Soviet troops since the 1979 invasion.

Hekmatyar, who was chosen spokesman for the seven groups, and other rebel representatives arrived here during the 40th anniversary commemoration of the United Nations to open a campaign to gain a seat in the General Assembly and oust the representative of the Babrak government.

It was the first time the leaders of the groups, which have frequently been at odds with one another, have joined in a common diplomatic front, they said.

"Right now, we are more united than we have ever been," said Abdul Raheem, representative of Jamiat-i-Islami, one of the groups.

Representatives of six groups participated in the two-hour interview here. They said they have met with representatives of more than a dozen U.N. missions here. Today they met for 45 minutes with U.S. Ambassador Vernon Walters.

In a letter delivered to U.N. Secretary General Javier Perez de Cuellar Thursday, the mujaheddin called on the world body to oust representatives of Afghanistan's "Soviet puppet regime." They asked the United Nations to call for the withdrawal of Soviet troops and send a fact-finding mission to Afghanistan to record "countless atrocities" they said the Soviet and Afghan government forces had committed against civilians, using napalm, dumdum bullets, poison gas, chemical weapons and cluster bombs.

Noting that U.N. resolutions have repeatedly condemned the invasion, Zabihullah Mojaddidy of the National Liberation Front said, "It is an amazing contradiction that the person who is the by-product of the very intervention that is condemned by the U.N. is sitting in that body. Shouldn't we the mujaheddin be the ones who should be recognized? There is no question that we represent the Afghan people."

The resistance leaders said they would make several visits to New York and talk to every one of the 119 countries that had supported them in a U.N. vote last year before the next General Assembly meets.

"It will be a long process," said Sididdiqu Saljoqui, representing the Mahazi Mil-i-Islami.

While U.N. resolutions, sponsored by Pakistan, condemn the invasion, they do so without mentioning the Soviet Union by name. Rose Berstein, a spokeswoman for the U.S. mission, said it would be an uphill battle for the Afghans to win a more specific challenge calling for a withdrawal of Soviet troops or the ouster of the Babrak government's U.N. representative because most nonaligned nations would side with the Soviet Union in such a vote.

The Soviet forces, according to Hekmatyar, spokesman for the seven, are continuing to "increase their forces, consolidate their positions, building new airports and barracks."

The mujaheddin hold 200 Soviet prisoners of war in different parts of the country, he said, adding that the Soviets have been unwilling to trade prisoners under the auspices of the International Committee of the Red Cross. A handful of Soviet prisoners have reached the West through mediators, but efforts to arrange a full-scale release have not succeeded.

Hekmatyar said the recent Soviet effort to close the borders to Pakistan had been unsuccessful, and heavy casualties had been sustained on both sides during the last two months, including about 150 mujaheddin killed and 800 wounded.