The United States said yesterday that it "is seriously concerned by a recent upsurge of Libyan military activity in northern Chad," and U.S. officials said Chadian dissidents, backed by Libyan air strikes, could launch a major offensive this week against Chad's president, Hissene Habre.

State Department officials said a move against Habre this week is "a reasonable assumption" because it would strengthen Libyan leader Muammar Qaddafi's efforts to block recognition of Habre's delegation by the Organization of African Unity, which is scheduled to meet in Addis Ababa on Monday.

Qaddafi is backing the forces of former president Goukouni Oueddei, who was ousted by Habre a year ago in the most recent round of the central African nation's 17-year civil war. Libyan troops have occupied the Aouzou Strip in northern Chad for a decade, and in 1980 they invaded the rest of the sparsely populated country in support of Goukouni.

In a statement, the State Department said it has information that Libya is increasing its air activity in the Aouzou Strip and deploying troops to its southern border "possibly for introduction into the fighting in Chad at an opportune time."

It added that, "while there is no evidence of Libyan preparations for a land invasion of Chad as in 1980, fighter aircraft, with support elements now in place at Aouzou, would be capable of launching attacks" against Habre's forces.

"The United States would consider Libya's military involvement in air support to be a serious military intervention in Chad and a major escalation of its involvement in the rebellion," the department said.

U.S. officials privately expressed concern that the activity in Chad might mark a renewal of Qaddafi's efforts to bring Libya's southern neighbors under his control.

His activities in that area have been restrained since February, when an alleged Libyan drive against Sudan caused President Reagan to dispatch the aircraft carrier Nimitz to Mediterranean waters near Libya and send four Airborne Warning and Control System (AWACS) radar planes to Egypt to maintain surveillance over the Libya-Sudan border area.

Despite their concern, the officials said there were no plans for any immediate U.S. actions except to maintain close contact with other interested countries. They acknowledged that these countries include France, which has provided Habre with military aid in the past; but the officials stressed that they have no knowledge of any French intention to intervene anew.

The officials noted that Habre's forces have fared poorly in recent clashes with the dissidents. As a result, they added, Qaddafi may have concluded that the time is ripe for a concerted new drive by Goukouni to topple Habre, with whom he has long been engaged in a seesaw battle for control of Chad.

Making Habre's position even more precarious has been a new outbreak of clashes with Nigeria in western Chad stemming from a two-decade border dispute between those two countries. Chad claims that in recent days Nigerian military planes have killed 98 Chadian civilians in bombing raids.

U.S. officials said that, despite suggestions of collusion between Nigeria and Libya, they believe the two outbreaks are unrelated. However, they added, the trouble with Nigeria is a serious problem for Habre because it has forced him to split his forces between two widely separated parts of his country and has impeded his ability to combat the Libyan-backed dissidents in the north.