LUANDA, Angola (AP) _ Rebels overran a government military base in northern Angola on Monday in a battle that killed a U.N. observer and caused many casualties on both sides, an Angolan general and a U.N. official reported.

Portuguese TSF radio said the rebels captured the base in Uige, about 150 miles north of Luanda, and also attacked Negage, 12 miles south of Uige and site of the northern air command.

Who controlled Negage was uncertain.

Gen. Higino Carneiro, the armed forces deputy chief, said both sides suffered heavy losses and heavy artillery was being used to try to drive the rebels from Uige.

The attacks came three days after both sides agreed at Namibe to a cease- fire and to uphold 1991 peace accords that were supposed to end this southern African nation's 16-year civil war.

U.N. special representative Margaret Anstee said Brazilian U.N. observer Adilson Barbosa Costa was killed either by a grenade splinter or mortar fire when he and four other U.N. observers were caught in cross fire as rebels fought police and armed civilians in Uige.

He was the first U.N. observer to die under fire here, she said.

The four other observers were evacuated Monday from Uige.

She called the attacks ''a big setback for the Namibe accords, but we have to try to make them work, because the alternative is unthinkable: either civil war ... or anarchy.''

In New York, the U.N. Security Council on Monday extended the mandate of U.N. peacekeepers and observers in Angola for two more months and urged both sides to fulfill the peace accords. Secretary-General Boutros Boutros-Ghali deplored the latest violence and expressed condolences over Barbosa's death.

Information officer Jorge Valentim of Jonas Savimbi's National Union for the Total Independence of Angola rebels said their side attacked in retaliation for the murder of rebels last week by government forces in Uige province.

But on Angolan state radio Carneiro called the attacks an unprovoked violation of peace accords.

Both sides were due to meet this week to arrange terms for the rebels to join a government of national unity and assume seats won in September parliamentary elections.

New Prime Minister Marcolino Mico said Monday night on state radio he would postpone anouncement of the new government, which was due Monday.

Savimbi's group won 70 seats in September elections. Savimbi rejected the results as fraudulent when the governing Popular Movement for the Liberation of Angola won a parliamentary majority. Savimbi lost in the first round of the presidential race to President Jose Eduardo dos Santos.

Fighting broke out Oct. 31, with up to 2,500 people killed in the capital alone. The United Nations has been trying to preserve a truce since then to prevent a return to all-out civil war.

Angola's civil war began on the eve of independence from Portugal in 1975 and killed 350,000 people.