President Reagan yesterday put sharp limits on the military aid the United States will give Chad in its war against Libyan-backed rebels and told reporters that Chad is "a sphere of influence of the French" and primarily their responsibility.

In a 14-minute question-and-answer session hours after the fall of the key Chadian town of Faya Largeau, Reagan said that the United States cannot play "world policeman," and appeared to be putting new pressure on the French to exert their historic influence in the region.

"As I've said before, it is not our primary sphere of influence," Reagan said. "It is that of France. We remain in constant consultation with them the French , but I don't see any situation that would call for military intervention by the United States there."

High administration officials said after the brief news conference that the same point had been made to the French through diplomatic channels.

"This was not so much a message to the French as it was a report to the American people saying that there are definite limits to what the U.S. government is prepared to do," one official said.

Reagan spelled out the most important of these limits when he said that the U.S. role as far as troops are concerned would be restricted to transporting African military forces that want to assist the Chadian government. He also referred to U.S. disappointment with the French for failing to provide air support for the airborne warning and control system (AWACS) planes that the United States has sent to Sudan.

"I don't know what their plans are," Reagan said. "Frankly, we had believed at first that there was going to be some aerial activity there."

At the State Department, spokesman John Hughes said, "Some of our people are conferring with the French about the possible uses of the AWACS."

According to administration sources, the United States had been led to believe that the French would provide air support for the AWACS and had sent the planes to Sudan on this basis. One official said yesterday that this is still the U.S. expectation.

The confusion about the AWACS underscored the difficulty that U.S. officials have had throughout the crisis in trying to determine French intentions and resolve in the Chadian conflict. Reagan yesterday said he didn't know whether France was negotiating with Libya and said he also wasn't "privy to their military planning."

The official explanation given for this lack of information is that the French are sensitive that their plans and strategy will become widely known if shared with American officials.

"I think that they know the more something is talked about the more chance there is of leaks, and the leaks in this case could benefit the wrong people," Reagan said.

Reagan's comments on the need for France to take the lead in Chad follow two weeks of quiet efforts by the administration to convince Paris of the urgency of the threat posed by Libya.

But the socialist government of President Francois Mitterrand has been reluctant to become too deeply involved in the defense of its former colony, arguing that a 1976 military cooperation treaty with Chad requires it to give only "logistical assistance."

Mitterrand, concerned that France once again might become involved in Chad's unending troubles, has sent 180 paratroopers as advisers, but has rebuffed requests from President Hissene Habre for fighter aircraft to counter Libyan bombing raids.

U.S officials publicly have praised France for sending $45 million in aid and emphasized, as Reagan did yesterday, the "close consultation" between the two countries, but privately they have complained that much more needs to be done.

The Pentagon, meanwhile, revealed that of $25 million in emergency U.S. aid earmarked for Chad, $9.7 million already has been spent. Of that, according to spokesman Army Brig. Gen. W.C. Cousland, $4 million has gone to pay for shipping equipment. Cousland said that Air Force C141 Starlifter transports have made 26 trips into Chad, 23 of them from the United States and three from Europe.

U.S. officials sought to play down the significance of the loss of Faya Largeau to rebels led by former president Goukouni Oueddei and his Libyan allies and pointed out that Habre's forces, according to U.S. information, had retreated from the northern oasis in an orderly way.

Reagan pointed out that, having taken Faya Largeau, the rebels and the Libyans are still 500 miles away from the Chadian capital of Ndjamena. But one official said: "Before we had Habre going forward and Goukouni going back. Now we've got Libyan leader Muammar Qadaffi and Goukouni going forward and Habre going back."

The president's remarks came on the eve of a 25-day trip that will be divided almost evenly between vacation at his Santa Barbara ranch and political speeches to Hispanic, veterans' and women's organizations.

White House officials often arrange brief question-and-answer sessions on the eve of vacation trips, partly to demonstrate that Reagan is a working president. Reagan's advisers also want to show that he is in command on foreign policy issues. The briefing of the president focused on Central America and Chad, according to officials.

When Reagan was asked whether it was America's role to play policeman in the world, he quickly responded, "No, it is not" and then added that the United States must recognize that threats to its security can be "widespread." He observed that the United States is dependent upon imports of oil and strategic materials.

In contrast to some of his appearances, the president appeared to enjoy the give-and-take at the brief press conference yesterday, but he was cut off after answering nine questions, all on foreign policy, by the prearranged rules laid down by spokesman Larry Speakes.