A U.S. military transport plane with 21 American personnel aboard crashed in bad weather this morning just off the Caribbean coast of Honduras, a U.S. military spokesman said.

The C130 was en route from Howard Air Force Base in Panama to Trujillo air base on the Honduran coast in "a routine airlift mission," the spokesman said. The U.S.-constructed base at Trujillo serves the nearby Regional Military Training Center, where U.S. Special Forces are instructing Honduran soldiers. There are about two or three such flights to the base each week, the spokesman said.

There was no official word on casualties this evening, but the accident appeared to be the costliest accident in the two years since the United States began major military maneuvers in this Central American country.

Until now, the total number of U.S. dead in Honduras since February 1983 was 14, a U.S. official said, with the highest single casualty toll being eight dead in a helicopter crash 11 months ago.

The four-propeller C130 Hercules went down in "very poor flying weather" at about 11 a.m. about 500 yards off Honduras' northern coast, the U.S. spokesman said. The rain and wind were so strong that it was impossible to launch either boats or planes to search for possible survivors, and the passengers' condition was "unknown," he said.

"No hostile action was indicated in the mishap," said the spokesman, reading from a prepared statement. "A board is being formed to investigate the accident."

In Washington, Pentagon spokesmen offered similar details. One noted that a U.S. Navy frigate was en route to crash site.

Another said the plane, which apparently overshot the Trujillo runway, was assigned to the Air Force Reserve 440th Tactical Airlift Wing based in Milwaukee. The spokesman did not know how many persons among the five-member crew and 16 passengers were from that unit.

The air base in Panama is one of the few remaining U.S. installations in that country.

About 1,000 U.S. military personnel are in Honduras, and the number if expected to rise in March with the start of U.S.-Honduran maneuvers to be called Big Pine III.

The maneuvers have served to provide training for both U.S. and Honduran troops, and to attempt intimidation of the left-wing government in neighboring Nicaragua.

Only one U.S. military man is known to have died from hostile action during the maneuvers here. He was a helicopter pilot shot to death by Nicaraguan militiamen a year ago, moments after an emergency landing landing just inside the Honduran border. His helicopter had strayed over Nicaragua and was shot down. The other victims in Honduras have died in accidents.

In neighboring El Salvador, a U.S. civilian pilot died on an apparent CIA intelligence mission that ended in a crash on the side of a volcano near San Salvador.

Almost two years ago, Army Sgt. Jay T. Stanley was wounded by gunfire while piloting a helicopter on a mission to establish an airborne communications link with a U.S.-trained battalion in El Salvador.