"The benchmarks of success that we've laid out are: Are we building an Afghan national army and police structure that can secure itself without the assistance of NATO forces or U.S. forces? Is Pakistan able to maintain its borders so that al-Qaida or affiliates aren't operating there?" Obama said.

Pakistan's army said Thursday it had moved troops from elsewhere on its side of the Afghan border to the stretch opposite Helmand to try to stop any militants from fleeing the offensive. Helmand's strategic setting will give the U.S. an opportunity to interdict fighters coming from Pakistan

Elsewhere, the U.S. military announced that insurgents were believed to have captured an American soldier in eastern Afghanistan on Tuesday. The missing soldier was not involved in Operation Khanjar, or "Strike of the Sword," under way in southern Afghanistan.

The southern offensive was launched shortly after 1 a.m. Thursday as thousands of Marines poured from helicopters and armored vehicles into villages along the Helmand River. Officials described the offensive, involving almost 4,000 newly arrived Marines and more than 600 Afghan security forces, as the largest and fastest-moving of the war's new phase.

The troops fanned out into the districts of Nawa and Garmser in central Helmand and up to 55 miles south in the vicinity of Khan Neshin, the capital of Rig district, according to the military.

Last summer, the 24th Marine Expeditionary Unit took the Garmser town and helped provide security for an area U.S. commanders say is now relatively secure. The U.S. would now like to replicate that success elsewhere in the province.

In Nawa village, Marines took militants by surprise by dropping behind Taliban lines, said Capt. Drew Schoenmaker, 31, of Greene, N.Y.

"We are kind of forging new ground here. We are going to a place nobody has been before," said Schoenmaker, who commands Bravo Company of the 1st Battalion, 5th Marine Regiment.

At 3 a.m., several hundred Marines took positions in a freshly plowed dirt field around Nawa. The soft, deep dirt proved challenging for troops weighed down with days' worth of water, food and gear. Many frequently stumbled.

At daybreak the Marines walked along tree lines, and at 6:15 a.m. the company took its first incoming fire, likely from an AK-47 along a tree line. The next three hours brought repeated bursts of gunfire and volleys of rocket-propelled grenades, sending deep booms across the countryside.

A small force of Afghan soldiers accompanying the Camp Pendleton-based Marines got into several scraps with an insurgent force of about 20 fighters firing from a mud-brick compound.

The Marines, the Afghan soldiers and their British advisers surrounded the compound from the east and the south.