LUANDA, Angola (AP) _ UNITA rebels returned to peace talks Monday to negotiate the final details of a treaty to end 19 years of civil war, but said the signing would be postponed until this weekend.

Fighting was reported in northern and central Angola even as preparations were underway in Lusaka, Zambia, for the signing ceremony, originally planned for Tuesday.

The civil war has ravaged Angola since independence from Portugal in 1975, killing more than 500,000 people and pushing the oil- and diamond-rich country on Africa's southwestern coast to the brink of starvation.

The peace agreement would lead to a cease-fire and the start of a power- sharing agreement worked out in nearly a year of talks mediated by U.N. diplomat Alioune Blondin Beye.

UNITA called off the truce last week after the government overran their stronghold in Huambo, 330 miles southeast of Luanda. But the rebels later agreed to sign, apparently seeing the treaty as the best way to salvage a share of government power.

UNITA and government delegates on Monday were discussing details of the cease-fire and security for UNITA leader Jonas Savimbi, said rebel spokesman Marcus Samondo in New York. He said both sides agreed to put off the signing ceremony until Sunday.

There was no immediate confirmation from the United Nations.

After a short-lived peace accord in 1991, Savimbi arrived in Luanda with a force of heavily armed bodyguards. This time, the government wants to limit that number and integrate UNITA security men into the national police.

But Samondo said he thought problems could be resolved by Sunday.

President Nelson Mandela of South Africa and U.S. Undersecretary of State George Moose were travelling Lusaka for the signing ceremony.

Moose told reporters in Luanda that the United States was ''deeply concerned'' by a continuing offensive by government troops, and particularly by their capture of Huambo.

Military sources said northern Uige, the last of Angola's 18 provincial capitals still under rebel control, was surrounded and government troops were attacking inside city limits.

Fighting was reported around Huambo. Savimbi made the city his headquarters after a siege last year that killed 80,000 people.

President Jose Eduardo dos Santos and Savimbi were expected in Lusaka on Monday.

But Samondo said Savimbi was still in Angola, awaiting the outcome of the talks. Dos Santos was in Luanda awaiting news from his delegation.

Western diplomats say the government is risking possible U.S. sanctions by continuing its attacks on UNITA.

Sen. Jesse Helms, R-N.C., a fervent supporter of UNITA is set to become the new chairman of the Senate Foreign Relations Committee.