Chadian President Hissene Habre and visiting Zairian President Mobutu Sese Seko warned today that a "massive" buildup by Libya in the north of this country would result in resumed fighting "within a few hours or a few days."

In a joint communique, the presidents described the current quiet in the war against the Libyan-aided rebels of Chadian ex-president Goukouni Oueddei as "tactical."

Thousands of Chadians gave Mobutu a jubilant welcome for his having sent 2,500 troops to help the government forces in the civil war.

Habre and Mobutu rode standing in the back of a jeep through the downtown streets with their bombed-out and bullet-pocked store fronts--the detritus of past episodes in the sporadic war.

Mobutu, wearing his leopard-skin hat, held a carved walking stick aloft to acknowledge the crowd's cheers. Habre, dressed in a white, ankle-length robe and white cap, gave an almost shy response to the attention.

Before moving to talks in the palace, Mobutu told reporters that he had "come to show that Chad does not stand alone and will be helped to regain its territorial integrity." The entire visit lasted two hours.

There has been an eight-day undeclared cease-fire in the war since the Libyan Air Force bombed the government-held northern town of Oum Chalouba and France began dispatching paratroops into the battle zone.

While Mobutu is the only African leader to send troops to help Habre, such conservative, French-speaking African leaders as Felix Houphouet-Boigny of Ivory Coast and Abdou Diouf of Senegal asked French President Francois Mitterrand to intervene against what they describe as a bid by Libyan leader Muammar Qaddafi to bring Chad under his control.

More than 1,000 French paratroops are now in Chad, most of them deployed in the northern battlefront towns of Arada and Biltine in the east and Salal in the west--areas south of rebel-held Faya Largeau. Both Habre's forces and the Zairian troops are in the north as well. Agence France-Presse has reported that more than 1,500 backup French troops are poised in the neighboring Central African Republic. United Press International quoted a French spokesman as saying 160 Legionnaires sent from there joined the French contingent in Abeche.

A western diplomatic source said the estimates of Libyan regular Army troops now occupying the oases of northern Chad has risen to about 3,200 from 2,500. There has been no credible estimate of the number of rebel troops fighting under Goukouni.

The rebels, with the reported assistance of Libyan planes, tanks and armored cars, took Faya Largeau from Habre's forces Aug. 10. Western diplomatic sources report that a third force, an unknown number of troops of Libya's Islamic Legion--drawn from Africans drawn to jobs in Tripoli--also is fighting alongside the rebels.

While Qaddafi has denied that his forces are in Chad, a western diplomat said they are now at the oases of Gouro, Ounianga Kebir, Bardai, Ougoui and Fada.

A western diplomat speculated that Mobutu brought the "perspective" of 11 African heads of state who issued a declaration Tuesday in the Congo asking the Organization of African Unity to call on all parties in the Chadian war to seek a cease-fire and "withdrawal of all foreign troops from Chad." Although Zaire attended, Mobutu did not support the declaration.