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KANDAHAR, Afghanistan (AP) _ Popping out of gullies, drifting past disguised as shepherds or walking boldly by, hostile forces cross back and forth before the Marines guarding the allied air base in southern Afghanistan.

It's when the scarved, robed men are at their most daring _ as they are this week, toting Kalashnikov rifles in plain view and seemingly counting heads among the Marines who face them _ that Marines brace in their sandbagged foxholes.

``It's when they start moving in and out that they're staging for an attack,'' Marine Cpl. Jason Gravem, 23, of San Diego, said Tuesday.

``They act like sheep herders, but these sheep herders carry radios and call stuff in,'' said Marine Sgt. Ethan Ramsey, 22, of White Plains, Mo.

``The weird part of it is, they can just appear in an instant,'' Ramsey said. ``There's got to be tunnels.''

Suspected al-Qaida or Taliban holdouts have already attacked coalition forces at Kandahar airport once during the past week. Gunmen in the arid scrub north of the runway opened fire Thursday as a C-17 screamed into the air carrying 20 detainees bound for Guantanamo, Cuba.

A heavily guarded dirt-wall compound at the base holds more than 300 alleged Taliban and al-Qaida figures bound for a U.S. prison in Cuba for investigation.

U.S. Marine officials have said they believe the two events Thursday _ the departure of the first detainees and the first fire on forces here _ were unrelated. But the U.S. military is taking no chances.

Air Force, Army and Marine service members sent their second batch of detainees off Sunday, closely flanked by Cobra attack helicopters, while U.S. Humvees rigged with 50-caliber machine guns and TOW missiles sped across the runway below.

On Tuesday, debris rained down on the young Marines in their sandbag bunkers as explosives experts destroyed caches of arms newly found on the runway. It was evidence, Marines said, that hostile forces were massing for another try.

Experts used TNT and other explosives to collapse a mud-walled house and a network of tunnels recently found in the area. The house was only about 100 yards from the base perimeter, and a few dozen yards from one of the areas of fire of last week's attack.

Bulldozers leveled other mud ruins in the area on Tuesday. Foot patrols searched under cover of light-armored vehicles armed with machine guns.

The U.S. military on Monday had found mortar fuses and rocket-propelled grenades in the house _ arms that weren't there on previous occasions, Marine spokesman Lt. James Jarvis said.

``They were coming in to use them for an attack of some sort,'' Jarvis said.

U.S. forces found the stash after seeing seven men carrying Kalashnikovs and rocket-propelled grenades across the parched dirt plain at sunset Monday.

Men had been crossing back and forth throughout the day, Marines guarding the perimeter said. The men moved slowly along the fence line, Gravem said. Some of the men even waved, Marines who saw them said.

Marine officials declined to reveal the rules of engagement for armed forces here. The officials say they don't want hostile forces to know just how far they can provoke the Americans without retaliation.

But rank-and-file Marines say the suspected Taliban out there already know.

``They know the Geneva Convention better than we do,'' Ramsey said. ``They go back and forth, and we can't do anything.''

Crisscrossing ravines in the shrapnel-strewn, mined plains help intruders get in and out, Marines say. They have upped security since Thursday's shooting and may expand the base's perimeter.

Jarvis said he knew of no legitimate civilian use of the area; villagers who lived in mud homes nearby have long abandoned them. He said the U.S. military was inclined to treat anyone in the area as suspicious.

Young Marines camp out night and day in the line of fighting holes along the runway. New sandbags have replaced ones pocked with bullet holes from the last attack.

``I don't underestimate them,'' Ramsey said. ``I think they might underestimate us. I want them to come back. They're taunting us now.''