U.S. officials say that rebel forces in Angola -- using sophisticated antiarmor and antiaircraft weapons supplied by the United States -- turned back the Angolan Army's biggest effort in 12 years of sporadic fighting to capture the rebels' headquarters.

In several weeks of fighting that subsided in mid-October, the rebels inflicted heavy losses on the Angolan Army, which is supported by Cuba and the Soviet Union, the officials said.

The success has strengthened administration support for its covert military aid program to the rebel forces of Jonas Savimbi's National Union for the Total Independence of Angola (UNITA), according to these officials.

Some senior U.S. officials suggest that the administration increase its annual $15 million in aid to reduce UNITA's dependence on South Africa, whose long-range artillery and air power played a role in turning back the latest government offensive.

The administration has been providing UNITA with Stinger missiles and this year sent TOW antitank missiles and other weapons to help it cope with the expected heavy use of armor in the Angolan Army's offensive.

At the same time, the administration has decided to take a tough stand on Angola's recent request for admission into the International Monetary Fund. Both the State Department and Secretary of the Treasury James A. Baker III wrote to Sen. Dennis DeConcini (D-Ariz.) in mid-October assuring him of U.S. opposition to Angola's IMF application -- at least until it reaches an agreement with Washington on withdrawal of Cuban troops and adopts "market-oriented policy reforms" in its economy.

The administration appears determined to keep up military and political pressure on the Luanda government, which has recently shown signs of flexibility in negotiations with the United States about a timetable for withdrawal of the 37,000 Cuban troops.

In a recent briefing, U.S. officials said Savimbi's forces seized from the Angolan Army "very substantial" quantities of recently delivered Soviet weapons -- including "dozens of tanks," armored personnel carriers, trucks "galore" and a few SA8 and SA13 antiaircraft missiles.

The rebels, with some South African artillery and air support, also "decimated" the Angolan Army's 47th Brigade, and "seriously beat up" three or four other brigade-sized units in the three-pronged attack on Mavinga, the gateway to UNITA's main stronghold in far southeastern Angola, according to the officials.

The fighting began in late August and ended in mid-October with the retreat of six to seven Angolan Army brigades to their main base at Cuito Cuanavale, 125 miles northwest of Mavinga.

The Angolan government spent two years preparing for the offensive, which U.S. officials said involved about 10,000 troops. The Soviet Union provided about $1 billion in arms, and Soviet and Cuban advisers also were more involved than ever before in planning and executing the offensive, according to these officials. However, no Cuban combat troops were involved, the officials said.

UNITA said it captured two Cuban pilots -- Lt. Col. Manuel Rocas Garcias and Capt. Ramos Kassadas -- whose MiG23 jet fighter the rebels shot down Oct. 28 over eastern Angola. It was not clear whether the rebels used a Stinger missile.

"We're going to present them {the Cubans} at a press conference in Jamba Nov. 11, the 12th anniversary of our independence from Portugal," said Marcos Samondo, a UNITA spokesman here. Jamba is the UNITA headquarters.

"We're going to show all the equipment that has been captured. We captured 20 T55 tanks in good condition and six SA8 and SA13 missiles," he added.

U.S. officials said Angolan government and South African press reports of South Africa's military involvement on the side of UNITA probably were exaggerated. But they said there is "little doubt" that South African artillery guns, as well as "some" South African aircraft, played a role in UNITA's victory.

In an Oct. 15 report, the Luanda government said it had shot down six South African Impala planes, three Mirages, four other unidentified planes, at least four helicopters and one light reconnaissance aircraft. It also said four regular South African battalions and its "Buffalo Battalion" of special forces crossed into Angola.

U.S. officials said they believe only one South African spotter plane and maybe one Mirage were shot down. They said both the South Africans and the Angolans and their Cuban allies, fearful of each other's improved air defense capabilities, were generally "less aggressive" in the air.

These officials say they are uncertain what role South African ground forces may have played in the fighting but think Angolan claims are probably exaggerated.

As it was preparing its offensive, Angola renewed negotiations with the United States over a withdrawal of Cuban troops and was showing a new flexibility on setting a timetable. Whether the failed offensive will affect its willingness to dispense with Cuban troops will not be clear until the next round of U.S.-Angola negotiations, U.S. officials said.

A delegation of top Angolan officials is now visiting Havana and Moscow for what are presumed to be consultations on this issue, as well as on the Cuban and Soviet willingness to continue to provide massive supplies of arms and assistance for further military offensives against UNITA.