CAMP BIG BEAR, Honduras (AP) _ National Guard troops from Missouri are building Camp Big Bear in mountainous northern Honduras as part of another joint military exercise that will bring about 5,000 guardsmen from nine states here.

A U.S. military offical said the outpost is a ''physical demonstration of U.S. commitment'' to the region. He spoke of possible combat if Nicaragua's left-wing government brought in Soviet jet fighters or Cuban troops.

''The image of American soldiers walking through villages down here (in combat) is abhorrent to me. A direct invasion would be a catastrophe for all involved. It would be successful, however,'' said the offical, who spoke to reporters at Palmerola Air Base in central Honduras where 1,100 U.S. military personnel are stationed.

According to the guidelines set for the news conference the official could not be further identified.

''To intervene in the internal affairs of Nicaragua is not in our best interests, but if they bring in MiG-23s or put a Cuban battalion in there, that's a different story,'' he said.

The guardsmen landed Saturday at Palmerola in four U.S. military aircraft and then came to Camp Big Bear, located 100 miles north of the capital of Tegucigalpa and about 150 miles northwest of the Nicaraguan border.

No more than 500 U.S. guardsmen will be in Honduras at any one time during the exercises that will continue into May, and they will serve for two-week periods, according to Alaska Air National Guard Maj. Carl Gidlund, the public affairs officer at Palmerola.

He said about 5,000 guardsmen from North Dakota, Arizona, California, Maine, Alabama and Missouri will participate in the U.S.-Honduran training exercise called ''Terencio Sierra '86.''

Some 200 guardsmen frm Arkansas, Pennsylvania and Michigan and 200 U.S. Army soldiers from Ft. Bragg, N.C., will participate in other artillery exercises beginning about Feb. 1, Gidlund said.

One major purpose of ''Terencio Sierra '86'' is to give the guardsmen, most of whom are combat engineers and heavy equipment operators, experience in building roads in a developing country, officials said. They are scheduled to improve a 15-mile stretch of a washed out road between the villages of Puentecita and Jocon.

The 34-foot-wide gravel road will become an extension of a similar road that U.S. forces built in this rugged area last summer.

Guardsmen also will drill four water wells for villages in the area and provide medical services for the residents, said Maj. Charles Friend, a Missouri guardsman who is Camp Big Bear's operations officer.

The Missouri troopers erected large canvas tents on the muddy valley floor that will be their home for the next two weeks.

''It'll be good to get to sleep - we've been on the go since Friday,'' said Capt. Harry Woehrle, 31, a mechanical engineer from St. Louis, as he threw his duffel bag into a tent.

U.S. officials said the joint exercises will not only provide training for American troops in a difficult environment but also will help buildup Honduras.

''If we fail to produce economic progress, the consequences are predictable,'' said the U.S. offical in Palmerola. He said Nicaragua's Sandinista government ''means to destabilize the region, to take advantage of its economic underdevelopment and then to impose military garrison states a la Cuba, and that's their plan.''

The U.S. economic assistance program to Honduras is the third largest in the Western Hemisphere. U.S. economic and military assistance to Honduras will total $214.7 million for the current budget year.

U.S. forces have been conducting military exercises in Honduras almost continously since 1982. Last year more than 5,000 American soldiers participated in maneuvers that included tank exercises less than three miles from the Nicaraguan border.