This is a dispatch from the Afghanistan reporters' "pool," a Pentagon-authorized system that allows a single journalist to file for all accredited news organizations.

CAMP RHINO, Afghanistan — U.S. forces fired mortars around their forward base in southern Afghanistan late Thursday to repel what officers said was "almost certainly" an attempt by enemy Taliban or Al Qaeda forces to probe their defenses.

[A UH-1N Huey helicopter crashed near the airstrip here at Camp Rhino, and Marine spokesman Capt. Stewart Upton said two servicemen received minor injuries, one of them on the ground.]

[He said the cause of the crash was under investigation, but "we are 99 percent sure that the helicopter did not crash because of enemy fire."]

Illuminating flares lit up the flat, dusty desert around the Camp Rhino base, while dozens of high-explosive shells let off solid thumps as they exploded.

Journalists in the camp, who were handed military-issue flak jackets and helmets and told to huddle in a trench with their soldier escorts, reported seeing no incoming fire.

But they heard yelling outside the camp and the sound of gunfire. Helicopters were also making sweeps in the clear night sky.

A spokesman at the base, Marine Captain Stewart Upton, said the activity was because enemies had been detected in the proximity.

He did not specify whether they were Taliban or Al Qaeda fighters.

"We're almost positive it is enemy probing," Upton said, adding that it was clear that whoever was near the camp was there with "hostile intent."

He said the enemy activity was continuing into the night.

Another spokesman, Captain David T. Romley, said the base was reacting to "a credible threat," and said lightly armored reconnaissance vehicles were being used for what was likely a "hunter-killer" mission against the intruders.

He explained that recon teams are often sent out to identify the location of enemy forces, which are then shelled with mortars.

U.S. Defense Department rules governing the journalists' presence in the camp forbid reporting on exact operational measures being taken.

The approximately 1,300 Marines at this sprawling base have since Monday begun staging deeper reconnaissance missions to the north in support of the Afghan opposition factions closing in on Kandahar.

Patrols have been mounted on foot, by armored vehicles, and by helicopter, partly with the aim of cutting off avenues of escape for the Taliban and Al Qaeda, U.S. military officers say.

They are also trying to block lines of communication and supply in areas south of Kandahar, but so far U.S. forces have not destroyed any enemy materiel since staging the deeper patrols, said Romley.

The complex of pre-existing buildings used by the Marines for their base — a collection of warehouses and watchtowers surrounded by concrete perimeter barriers — has been alternatively called either a hunting "lodge" for wealthy Arabs or a drug distribution center.