NUREMBERG, West Germany (AP) _ NATO's winter maneuvers ended a week early on Friday due to mild weather that made the ground too soft for heavy tanks. Thousands of military vehicles swarmed onto the highways, returning to their depots.

The maneuvers, dubbed ''Certain Sentinel '86,'' began Monday and were scheduled to end Jan. 31. They were part of the annual North Atlantic Treaty Organization exercise known as Reforger - the Return of Forces to Germany.

U.S. Army Lt. Gen. Andrew P. Chambers, the exercise director, terminated the maneuvers a week early for safety reasons and to minimize damage to the soggy German countryside.

More than 70,000 U.S., Canadian and West German troops were dispatched to northern Bavaria for the maneuvers, but a full-sized field exercise was never begun due to unseasonably mild, rainy weather.

The troops included more than 20,000 U.S.-based soldiers airlifted to Europe in Reforger. Chambers said the U.S.-based units would stay through the end of the month and begin returning home on Feb. 1 as scheduled.

''About 5,000 vehicles will be on the road Friday and Saturday to various places in West Germany for equipment cleaning and turn-in at assembly areas,'' said Army spokesman Capt. Steven Hatcher.

West German police listed 240 traffic accidents involving military vehicles and 53 people injured since the start of Reforger three weeks ago.

The Army said there were 33 ''reportable'' accidents during the five days of maneuvers. An accident is reportable if it causes more than $700 damage or loss of one day's work due to death or injury, according to the Army.

Four U.S. soldiers and a West German civilian were killed in accidents associated with the maneuvers.

Two of the soldiers were killed on Thursday aboard an Army helicopter that crashed in a densely wooded area of northern Bavaria after leaving the maneuvers. Two others were killed and four injured Jan. 17 when two Army helicopters collided in the air.

Bavarian Governor Franz-Josef Strauss applauded Chambers' decision to cut short the maneuvers.

''The farmland and forest owners feared for large-scale maneuver damage, had the exercise continued as planned. Your courageous decision has prevented such an extreme case,'' Strauss said in a telex. ''I would like to express my heartfelt gratitude and respect to you and your officers for taking this step.''