WASHINGTON (AP)—The Pentagon ordered fresh infantry, aerial gunships and top-of-the-line tanks to Somalia Monday to bolster U.S. forces after at least 12 Americans were killed, 78 wounded and others captured in the fiercest fighting since the mission began.

The casualties, inflicted by the forces of warlord Mohamed Farrah Aidid, were the most for the United States in such a short period since the Persian Gulf War. The battle began late Sunday and stretched into Monday.

On Capitol Hill, some lawmakers called for U.S. withdrawal, but President Clinton insisted American forces would remain until order was restored.

Reinforcements were being sent, he said, because ‘‘I’m not satisfied that the folks that are there now have the protection that they need.’‘

Disturbing pictures of dead and captured American soldiers filtered back from Mogadishu. Television footage showed a frightened, wounded soldier being questioned by his captors—prompting a warning from Clinton.

If any U.S. soldiers are mistreated, he said, ‘‘the United States—not the United Nations—will view this very gravely and take appropriate action.’‘

Pentagon officials declined to specify the number it believes are being held.

The captured soldier identified himself as Army Chief Warrant Officer Mike Durant, a Blackhawk helicopter pilot. The Pentagon said his unit was based at Fort Campbell, Ky.

A gruesome photograph showed cheering Somalis dragging by rope the body of an unidentified American through the streets.

Defense Secretary Les Aspin announced at the Pentagon that a mechanized company of Army soldiers with an armored platoon was being dispatched to Mogadishu to reinforce the U.S. force there. The company will be composed of about 220 soldiers, officials said.

The Army company will be equipped with heavy armor, including four M1-A1 battle tanks and 14 Bradley fighting vehicles, which are armored troop carriers, Aspin said.

Aspin said the Pentagon also was sending two AC-130 gunships, replacement helicopters and 200 other troops to replace the wounded and others due to leave. There are about 4,700 U.S. troops in Somalia.

At the State Department, spokesman Mike McCurry said the administration remained ‘‘firmly committed’‘ to its mission in Somalia, which began last December as a humanitarian venture but took on a more military style as the U.N. became determined to arrest Aidid.

A U.N. official in New York said the two men who were most wanted after Aidid were captured in the battle.

Clinton expressed his condolences for the families of those killed and his determination to see the mission through.

‘‘They were working to assure that anarchy and starvation do not return to a nation in which more than 300,000 people had lost their lives, many of them children, before the United States led the United Nations mission there starting last year,’‘ he said of the U.S. soldiers. ‘‘They were acting in the best spirit of America.’‘

Senior military leaders, briefing reporters on the condition of anonymity, said most of the U.S. casualties occurred when the Americans came under heavy fire from several hundred Somalis after they raided a meeting of Aidid supporters at a hotel. Two Blackhawk helicopters attempting to aid the soldiers were shot down and surrounded by opposing forces.

As for the new armor being sent in, the officers said the tanks will be used to clear roadblocks, while the armored vehicles and aerial gunships have night-vision scopes that will allow them to operate at all hours.

One of the military officers said the reinforcements were needed because Aidid’s men have become more capable after honing their guerrilla-type tactics against the U.N. forces.

The fighting came as United Nations forces launched a new attempt to destroy Aidid’s military command structure in Mogadishu.

The fresh troops will come from the 24th Infantry Division (Mechanized), based in Fort Stewart, Ga.

The forces were expected to be flown from Hunter Army Airfield near Fort Stewart to Somalia aboard C-5A transport planes.

The M1-A1 tanks to be sent to Mogadishu will be specially outfitted with plows to detonate mines. Such heavy weaponry hasn’t been available to the U.S. forces on the ground in Somalia since the Marines left several months ago.

Clinton said the United States must not waver from its commitment to help erase ‘‘brutality and anarchy’‘ in the East African country.

Even so, the latest casualties sparked new calls on Capitol Hill for the administration to take another look at its policy in Somalia.

‘‘Americans by the dozen are paying with their lives and limbs for a misplaced policy,’‘ said Sen. Robert C. Byrd, D-W.Va.

And Senate Minority leader Bob Dole, R-Kan., said: ‘‘It seems to me it’s time to take a hard, hard look on why we’re still there. … It’s gone from a humanitarian mission to almost an outright armed conflict, and it seems to me Congress and the administration ought to come to grips with this and make a decision one way or another.’‘

In Mogadishu, chief U.N. military spokesman David Stockwell confirmed that some U.S. troops were unaccounted for.

From the beginning of the Persian Gulf air war on Jan. 17 until the end of the ground war against Iraq on Feb. 28, the U.S. military counted 148 troops killed in action and 458 wounded.

The Iraqis scored a direct hit with a Scud missile attack against a U.S. Army barracks in Dhahran, Saudi Arabia, on Feb. 25, killing 28 troops and injuring 100.