Afghan resistance leaders appealed to President Reagan yesterday to break diplomatic ties with the Soviet-backed regime in Kabul and extend full recognition to the rebels, but Reagan said it would be "premature" to do so now, according to the leaders and White House officials.

Presidential spokesman Larry Speakes said Reagan believes U.S. representation in Kabul, the capital of Afghanistan, "plays a role in our efforts to be helpful to the resistance and our efforts to continue working with the Afghan people" against the Soviet occupation.

At a news conference held by the resistance leaders, Sen. Gordon J. Humphrey (R-N.H.), a leading supporter of the Afghan resistance cause in Congress, urged Reagan to break ties with the Kabul regime. He said it was "unfortunate and counterproductive" for the United States to maintain an embassy in Kabul while supporting the resistance.

Humphrey and Rep. Charles Wilson (D-Tex.) said Reagan should support the establishment of a government-in-exile by an alliance of resistance groups and push to have it seated at the United Nations in place of the Kabul government.

But the spokesman for the resistance leaders, Burhanuddin Rabbani, said the group does not intend to announce the establishment of such a government-in-exile during its visit here and will try instead to set up a government in rebel-held areas inside Afghanistan. "If this is not possible we will announce a government-in-exile," he said.

Another leader, Sebqatallah Mojadedi, said one key factor in deciding whether a rebel government could be established inside Afghanistan is whether the resistance received sufficient anti-aircraft weapons to protect the area of its bases from bombing raids.

According to Speakes, the leaders appealed to Reagan for "effective weapons for air and ground defense," describing Soviet attacks on Afghan villages. Speakes said Reagan has "pledged to support" the resistance, but would not comment further on the extent of U.S. aid.

Other officials have said the United States provides about $500 million a year to the resistance, called "freedom fighters" by Reagan.

There have been unconfirmed reports circulating in Afghan resistance support groups here that the United States has sent 300 shoulder-fired Stinger antiaircraft missiles and 150 launchers to the rebels. Recent news reports have suggested that the first batch of Stingers has arrived in Pakistan and the rebels' first attempt to use them resulted in only one hit in 12 attempts.

But one resistance leader, Ahmad Gailani, said the rebels had not received the promised Stingers. "If we had received them, I would have mentioned it," he said. Rabbani sidestepped questions about military aid and whether it was getting through to the resistance. "We do not know exactly how much is being provided and therefore we cannot say how much of it is getting through," he said.

Rabbani and the others are leaders of the newly formed Alliance representing seven resistance factions and at least 50,000 guerrilla fighters.

In a statement following the 35-minute White House meeting, Reagan reaffirmed administration support for the resistance. "It is an unshakeable commitment," he said. "Your goal is our goal: the freedom of Afghanistan. We will not let you down." Reagan also expressed hope for a "negotiated end to this war," but said "only an agreement that has the support of the Afghan people can work," and added that the Alliance "is the true representative" of those people.

Reagan also noted that the people and government of Pakistan have carried "a large share of the burden of Soviet pressure and intimidation" of the Afghan rebels.

Also yesterday, Reagan resumed his campaign for $100 million in aid to the counterrevolutionary rebels, or contras, fighting the leftist Sandinista government of Nicaragua. Reagan accused the Sandinistas of waging a "major attack . . . unprovoked assault" on Miskito Indian villages recently. "This bloody operation triggered a panic that led to an exodus into Honduras of nearly 11,000 villagers," he said.

Reagan displayed a photograph of a Soviet photo reconnaissance plane that the White House contends is operating out of Nicaragua, saying it gives the Sandinista government a "significant advance in its military and intelligence capabilities."