President Reagan has authorized $15 million more in emergency military aid to help the government of Chad fight an intensifying war against Libyan-backed insurgents, the State Department announced last night.

The money, which comes from a special $75 million fund set aside for emergencies under the Foreign Assistance Act, is in addition to $10 million approved by Reagan July 18.

The new disbursement, which does not require congressional approval, reflects growing administration concern about Libyan military and logistical support for rebels fighting the government of President Hissene Habre.

Reports from Chad yesterday said that Libyan warplanes had extended their bombing raids deeper into the country 200 miles south of the previous focus of fighting around the strategic oasis town of Faya-Largeau.

"The $15 million," the State Department said, "will be used to provide the government of Chad with a reasonable chance to defend itself against Libyan escalation." It was not yet possible to specify what the money would be used for, as the situation was "fluid."

"No one," the statement said, "has ever been very successful at forecasting the actions" of Col. Muammar Qaddafi, the Libyan leader.

Reagan earlier yesterday told President Mobutu Sese Seko of Zaire, who has sent 1,800 troops and six aircraft to Chad, that the United States was prepared to take additional steps to bolster the Habre government. Reagan later described their 30-minute meeting in the Oval Office as a "warm and useful discussion."

Reagan and Mobutu "agreed it is in their interests, the interests of stability in Africa and in our interest not to see Libyan aggression against an African state succeed," a senior administration official told reporters.

As the Zaire leader left the White House, Reagan said that the aircraft carrier USS Eisenhower would remain in waters off the Libyan coast despite a threat by Qaddafi to destroy American ships that enter the Gulf of Sidra.

"We will hold maneuvers as we always have in international waters," the president said. The United States and "the rest of the world" recognizes the gulf as international, and does not accept Qaddafi's claim to it, he said.

On Monday two F14 fighters from the Eisenhower were involved in an incident with two Libyan MiG jets. The Pentagon has also sent two airborne warning and control system (AWACS) aircraft to Egypt to observe Libyan aircraft movements in the area.

The United States has already sent 30 Redeye and Stinger antiaircraft missiles to Chad to help boost defense capabilities against Libyan bombing raids. Three U.S. Army sergeants from Fort Bliss, Tex., arrived there Wednesday to help instruct the Chadians on their use.

The State Department said last night that of the first $10 million sent to Chad, $7.5 million had already been spent and, "With the current level of activity and the fluid situation, the remaining balance will be quickly spent."

Last night's statement said that when the $10 million was authorized it was sufficient, together with aid provided by France, to enable the Chadians to prevail over the rebel forces, and to retake several strategic towns.

"But since that time," it continued, "Libya has initiated the large-scale bombing of Chad forces. This radically altered the situation and created the need for additional urgent military assistance."

The State Department is aware of Chad's exposure to the Libyan attacks, and senior officials say that they have been urging the French government to provide aerial cover for Habre's forces.

The French, however, who ruled Chad until independence in 1960, say that a 1976 military cooperation treaty with Chad does not oblige them to defend the country.

They have also told Washington, according to diplomatic sources, that the socialist government of President Francois Mitterrand feels that direct French military intervention could damage Habre's standing.

Officials say that U.S. intelligence reports indicate a constant resupply from Libya of arms and ammunition to the rebel forces of former president Goukouni Oueddei. Aircraft capable of airlifting insurgents into government-controlled areas as well as those involved in bombing raids have been observed on both sides of the Libya-Chad border.

Reagan said yesterday that he had expressed his "admiration" for Mobutu's "courageous" action in sending troops to Chad. He described the Zairean leader as "a faithful friend for some 20 years."

"On the home front," Reagan said, "the president has informed me of progress in his economic stabilization plan . . . . Zaire has taken the difficult but necessary steps to assure sustained economic progress and it's important that we and Zaire's other friends do what we can to help."

Mobutu praised Reagan for his "warm welcome," and said: "Some decisions have been made for economic aid to Zaire."