A group of conservative Republican congressmen has sued the Export-Import Bank to try to block $96 million in loan credits and guarantees to Angola on the grounds that it is a communist country.

The suit, filed in U.S. District Court in Alexandria, asks that the bank be stopped from disbursing any more funds under the loan package approved in June 1984 for a major offshore oil project in Angola.

The plaintiffs contend that under Ex-Im Bank rules, written by Congress, it is illegal for the government agency to deal with communist governments.

The administration and Congress are debating whether the United States should aid the Angolan rebel movement, UNITA, which is fighting to overthrow Angola's ruling party which is supported by Cuban troops.

"It is ludicrous for the Export-Import Bank to aid in the financing of a project for a communist nation while President Reagan publicly embraces that nation's rebel leader," Jonas Savimbi, said Rep. Philip M. Crane (R-Ill.), one of the six congressmen listed as plaintiffs in the case.

The other congressmen include Reps. Danny L. Burton (R-Ind.), James A. Courter (R-N.J.), Jack Fields (R-Tex.), Mark D. Siljander (R-Mich.) and Gerald B. Solomon (R-N.Y.).

UNITA leader Savimbi visited Washington this week to seek military aid for his struggle.

Reagan has approved a covert assistance program to Savimbi covering intelligence-sharing and other nonlethal aid. But as part of its "constructive engagement" strategy in southern Africa, the administration also has sought good relations with the Marxist MPLA, offering it economic aid through the Ex-Im Bank.

"As you know, there is a controversy over what the policy to Angola should be," said a Republican staff member of the House Committee on Foreign Affairs. The suit, he said, "is an attempt of the political right to get their licks in against 'constructive engagement.' It's a policy in shambles."

"We haven't yet been served with that suit," said Russ Boner, vice president for public affairs for the Ex-Im Bank. "Our position has been, and will continue to be, that Angola is not listed as a communist country" under the legislation which set up the bank in l961.

"Nor is it the administration's position" that it is a communist country, he said. The oil project being financed is a joint one between Sonangol, Angola's government-owned oil company, and the Cabinda Gulf Oil Co., he said.

Also listed as plaintiffs in the suit is the National Front for the Liberation of Angola (FNLA), and two of its officials, Holden Roberto and Jose C. Rebelo, who lives in Fairfax County.

Asked about the FNLA's participation in the suit, a Republican congressional aide said that Roberto's "credentials as an anticommunist are more persuasive than Savimbi's."