NAWA, Afghanistan (AP) - U.S. Marines suffered their first casualties of a massive new military campaign Thursday as they engaged in sporadic gunbattles along 55 miles of Taliban-controlled heartland in southern Afghanistan.

One Marine was killed and several others were injured or wounded on the first full day of the assault, the largest military operation in Afghanistan since the fall of Taliban government in 2001.

The offensive will test the Obama administration's new strategy of holding territory and letting the Afghan government sink roots in Helmand province. The insurgency has proven particularly resilient in this area, where foreign troops have never before operated in such large numbers.

President Barack Obama told The Associated Press in an interview Thursday that he has a "very narrow definition of success when it comes to our national security interests" in the region. "And that is that al-Qaida and its affiliates cannot set up safe havens from which to attack Americans."

(AP) U.S. Marines from the 2nd Marine Expeditionary Brigade, 1st Battalion 5th Marines board helicopters... Full Image

An immediate goal, the military says, is to clear away insurgents before the nation's Aug. 20 presidential election. Southern Afghanistan is a Taliban stronghold but also a region where Afghan President Hamid Karzai is seeking votes from fellow Pashtun tribesmen. Without such a large Marine assault, the Afghan government would likely not be able to set up voting booths to which citizens could safely travel.

The Pentagon is deploying 21,000 additional troops to Afghanistan in time for the elections and expects the total number of U.S. forces there to reach 68,000 by year's end. That is double the number of troops in Afghanistan in 2008 but still half as many as are now in Iraq.

Even bigger challenges, perhaps, will come in the weeks and months after the Marines have established their presence here.

The U.S. will have an opportunity to help develop alternate livelihoods for farmers whose opium poppy crops bankroll the Taliban. Helmand province is the world's largest opium poppy-producing area.

(AP) U.S. Marines from the 2nd Marine Expeditionary Brigade, 1st Battalion 5th Marines wait at Camp... Full Image

He also said Washington and its allies must build up the Afghan national army and police and help Afghanistan and neighboring Pakistan secure their common border.

"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.

(AP) U.S Marines from the 2nd Marine Expeditionary Brigade, 1st Battalion 5th Marines climb on to a... Full Image

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.

(AP) U.S. Marines from the 2nd Marine Expeditionary Brigade, 1st Battalion 5th Marines rest along a tree... Full Image

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.

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.

(AP) Map locates Helmand province, Afghanistan, where a U.S. offensive is underway Full Image

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.

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