Heavy fighting broke out in southeastern Angola this week between U.S.-armed rebels and forces of the Soviet-backed Angolan central government even as a top State Department official was visiting Angola for talks on possible withdrawal of 37,000 Cuban troops stationed there.

Preliminary reports reaching the U.S. government indicate the Angolan government offensive, involving six Cuban- and Soviet-supported Army brigades, had forced a rebel retreat of up to 60 miles toward Mavinga, a town in the last major line of defense protecting main rebel headquarters at Jamba.

The Angolan Army force numbers 4,000 to 5,000 troops, at least 28 tanks and 350 armored vehicles and trucks. The rebels, led by Jonas Savimbi's National Union for the Total Independence of Angola (UNITA), has amassed a guerrilla force of nearly equal size to block it, according to UNITA and Pentagon sources.

The fighting has taken on the aspect of a clash between two traditional armies rather than a guerrilla-style war, they said, and appears to be one of the largest battles since the civil war began after Angola won independence from Portugal in 1975.

The United States earlier this summer sent UNITA U.S. wire-guided TOW antitank missiles and 106mm recoilless cannons, usually mounted on jeeps or trucks, in anticipation of an armor-led offensive by the Angolan Army. UNITA has also received French antitank weapons, including the Milan and new Apilas missile.

The American weapons were part of a $15 million, CIA-managed covert aid program for UNITA this year that has included sophisticated Stinger antiaircraft missiles.

U.S. intelligence officials who recently reviewed the Angolan war situation and Savimbi's U.S. military aid program concluded he has sufficient arms to stave off a full-scale government offensive, according to informed sources. Whether the new battle will change this evaluation remains to be seen.

Savimbi's defenses at Mavinga, an old formerly Portuguese colonial town largely destroyed in earlier warfare, are reported to include World War I-style perimeter trenches. Loss of the town would be a severe blow to UNITA and endanger Jamba, which lies about 200 miles to the southeast, UNITA spokesmen say.

The new Angolan Army push to capture Mavinga apparently began just as Assistant Secretary of State Chester A. Crocker was making a surprise visit to the Angolan capital of Luanda for a new round of talks Tuesday and Wednesday on a possible partial withdrawal of Cuban troops from that country.

In early August, the Angolan government, after consulting with Cuba, modified an offer to withdraw all Cuban troops from the southern part of Angola and send home 20,000 over a three-year period. This action would be linked to independent elections in neighboring South Africa-administered Namibia.

The Angolans are now reported to have shortened the timetable to two years but are still insisting on the right to keep some Cuban troops in the capital and northern Angola.

The Reagan administration has previously said all Cuban troops must leave before it will establish diplomatic relations with the Marxist Luanda government.