Temperatures exceeding 100 degrees slowed down the movement of some of the troops, who dismounted from convoys to press ahead on foot in body armor, laden with heavy packs.
"There has been sporadic fighting, but so far no heavy engagement," said Capt. Bill Pelletier, a spokesman for the Marine Expeditionary Brigade- Afghanistan, which is based at Camp Lejeune, N.C. "We've had individuals or several people fire on our forces, but they broke contact pretty quickly once they gained the Marines' attention."
The attacking force also appeared mindful of new instructions from Army Gen. Stanley A. McChrystal, the commander of NATO and U.S. troops in Afghanistan, to make it a priority to avoid civilian casualties. Troops have been told that even if they believe insurgents have taken shelter in a particular location, they should refrain from bombardment if there is a possibility that civilians might be present, unless they themselves are in immediate danger.
Pelletier said the Marines so far had "not used indirect fire at all in our operations -- no artillery or bombs from aircraft."
The operation is likely to be a sustained one, involving the taking and holding of territory where thinly deployed British troops have mainly engaged in hit-and-run encounters with the insurgents.
The apparent capture of an American soldier in the eastern part of the country adds a complicating factor to this new phase of the Afghan conflict, which was driven by a fresh counterinsurgency strategy crafted by the Obama administration.
The number of U.S. troops in Afghanistan will reach 68,000 by year's end, reflecting a shift in priority away from the conflict in Iraq.
The Reuters news agency quoted a senior Taliban commander, Maulvi Sangeen Zadran, as saying the soldier was seized after leaving a base in Paktika province. Sangeen is allied with the Haqqani network, one of the most virulent of the Pakistani-based insurgent groups active in the border zone.
News accounts quoted Sangeen as saying that video of the captive, together with demands for the release of insurgent prisoners, would be forthcoming.
Abductions of aid workers, journalists and Afghan nationals are not unusual, but a military official said the U.S. soldier's apparent capture was believed to be the first of its kind in the course of the Afghan conflict.
The soldier's disappearance could raise embarrassing questions about an unauthorized departure from what are supposed to be heavily fortified bases where comings and goings are closely monitored.
