President Reagan yesterday authorized an "urgent" $10 million airlift of U.S. military supplies for Chad on grounds that the African nation and its neighbors are threatened by a Libyan-backed insurgency.

The American airlift, expected to begin before the end of this week, is reminiscent of a similar emergency supply of food and military equipment to Chad in November, 1981.

On that occasion, however, the U.S. supplies were helping to shore up then-President Goukouni Oueddei against Libyan-supported insurgents.

Since then, his forces have fled the Chadian capital of Ndjamena and, in a characteristic Chadian turnabout, are the main element of the Libyan-backed insurgency that the current U.S. program is intended to combat.

State Department officials said the new U.S. effort is essentially in support of France, which has airlifted 400 tons of war materiel and sent military advisers to Chad, and of Zaire, which is reported to be sending 2,000 paratroops and a half-dozen combat planes.

The beneficiary of the current airlift is the Chadian government of Hissene Habre, a radical Moslem leader reported to have been supported by an undercover U.S. effort in an earlier phase of a complicated power struggle.

Lawmakers were told that Reagan authorized the new aid because of "massive Libyan support" for the current challenge to Habre.

If Chad falls to a Libyan-backed insurgency, administration officials said, the neighboring government of Sudan would be threatened and Egypt gravely concerned.

The initial U.S. supply will be of clothing, foodstuff, Jeeps and other vehicles from existing military stocks, officials said.

Small arms and other weaponry may be supplied later, if needed, according to congressional sources.

Some of the supplies will be furnished directly to the Habre government, while some may be furnished to Zairian forces supporting Habre, officials said.

The $10 million effort, which includes the cost of U.S. airlift to rush the supplies to Chad, is being financed from a set-aside fund for use in emergencies, lawmakers were informed.

The emergency authority was also used in the 1981 U.S. airlift of supplies for the all-African peace-keeping force then backing the Goukouni government.

The Chadian civil war, which began 18 years ago, has involved 11 different factions with at least six separate military forces. All have been supported at one time or another by Libya, according to U.S. experts, as coalitions and power relationships shifted constantly.

State Department sources said a main U.S. policy line has been to encourage a leading role by France in dealing with its former colony, which became independent in 1960.

French forces have been called in by various Chadian governments since then, but Paris has rejected pleas from the Habre government to send combat forces.

The war materiel recently sent from Paris is credited by U.S. officials with helping to turn the tide of the latest phase of Chadian warfare. After suffering major reverses this spring, Habre's forces have retaken strategic towns in Eastern Chad earlier seized by Goukouni's troops.

Dependents of U.S. officials in Chad were evacuated from the country several weeks ago, and private Americans were advised to leave, State Department officials said.