Angola's Marxist government is not prepared to enter a coalition with noncommunist opposition forces led by Jonas Savimbi, an Angolan envoy said yesterday, warning the Reagan administration that it would be backing "a loser" if it decides to provide him with military or other assistance.

A national coalition government is "out," Angolan Foreign Trade Minister Ismael Gaspar-Martins said, because Savimbi's National Union for the Total Independence of Angola (UNITA) is regarded by the government as "an offshoot of South Africa" and is not a "national" movement.

"It's like telling you, 'All right, let's form a world government, bring Mr. Gorbachev and Mr. Reagan together and let them create a national government.' It cannot be done," Gaspar-Martins said.

The Reagan administration has been seeking a reconciliation between the Angolan government and UNITA as part of an overall settlement of various conflicts in southern Africa.

In addition, several senators have indicated that their decision on whether to back an administration proposal for military and other aid to UNITA would depend partly on the government's willingness to negotiate with Savimbi.

Gaspar-Martins, a foreign-policy adviser to President Jose Eduardo dos Santos, said his visit only a few days before Savimbi's arrival here to lobby for U.S. military assistance was purely a "coincidence."

But he went out of his way to make known his government's views on the congressional debate under way over the administration's proposal to begin providing covert aid to UNITA.

Gaspar-Martins warned that such a U.S. commitment to Savimbi is bound to have a negative impact on U.S.-brokered negotiations between his government and South Africa for withdrawal of Cuban troops from Angola and independence for South African-administered Namibia. But he did not say Angola would break off the talks.

He said Angola, in the latest discussions with Assistant Secretary of State Chester A. Crocker, has renewed an offer to withdraw Cuban troops from the southern part of the country and send most of them home but is not ready to make more concessions.

Angola is awaiting word from Crocker, who has just returned from South Africa, as to whether South Africa has new proposals regarding its withdrawal from Namibia and a schedule for independence elections there.

Gaspar-Martins also sought to dissuade Congress and the administration from making a commitment militarily or politically to Savimbi. He said UNITA forces have repeatedly engaged in the kind of terrorist activities against civilian targets in a guerrilla war against the central government that the Reagan administration has been condemning elsewhere.

He noted that, at its summit last summer, the Organization of African Unity criticized Savimbi's movement and U.S. proposals to begin aiding it. U.S. aid would be condemned as "a very unfriendly act not only against Angola but the rest of Africa," he said.

"I think the United States is backing the loser. Savimbi is a loser. He cannot win," Gaspar-Martins said.

"To us, you know, he is called 'Judas' Savimbi because of his treason, linking himself to South Africa," he said.