Before the mission, Schoenmaker, the company commander, said he would practice "tactical patience" as a way to avoid civilian casualties — an issue newly arrived Gen. Stanley McChrystal has underscored in recent weeks. Although troops in many similar circumstances have called in airstrikes on militant-controlled compounds, Schoenmaker did not.

"We made the decision to isolate the compound and not destroy it because we couldn't confirm if civilians were inside," he said. The militants were believed to have escaped out the back.

A Cobra helicopter circling overhead for most of the day fired rockets at a tree line nearby. Other Marines walked through fields of corn and hay. Only a handful of villagers dared to venture outside in the area of crisscrossing canals, mud houses and lush tree-lined fields.

The military said it had no confirmed reports of civilian casualties or damage to property and pointed out that it did not use artillery or other indirect fire. No bombs were dropped from aircraft, it said. There was no information about casualties among militant fighters.

Helmand's temperatures of well over 100 degrees Fahrenheit proved to be another enemy for the Marines. Because the troops were on foot, they had to carry all their own water and food. Forward observers and snipers spent the entire day under the cloudless sky.

"It's like when you open up the oven when you're cooking a pizza and you want to see if it's done, you get that blast of hot air. That's how it feels the whole time," said Lance Corp. Charlie Duggan Jr., 21, of Baldwinsville, N.Y.

But the Marines trained for months in the heat of the Mojave desert for the deployment, and many appeared happy to be here.

At one point, some 50 Marines were relaxing in an abandoned and dilapidated mud brick compound, their dusty-brown uniforms stained with perspiration. Someone spotted an Afghan male who appeared to be observing them from a nearby road.

The Marines quickly threw on their flak jackets and Kevlar helmets.

"It sucks, but it's what you've been training for your whole life," Lt. Chris Wilson, 25, of Ramsey, N.J., said with a smile as he held a radio with an eight-foot antenna. Thursday was Wilson's first mission into a combat zone.

Associated Press writers Jennifer Loven in Washington, Fisnik Abrashi in Kabul and Nahal Toosi in Islamabad contributed to this report.