Three U.S. soldiers died after a roadside bomb attack in southern Afghanistan on Saturday, NATO said in a statement.

No details were given. U.S.-led troops launched a NATO offensive on Saturday in the Taliban's last major stronghold in Helmand. It was not clear whether the three were killed in that campaign.

U.S. Marines launched a massive assault to take Marjah depite large number of civilians, in the first major show of force since Obama ordered 30,000 extra troops.

Within hours of the operation getting underway, U.S. Marines attacked on Taliban fighters in Marjah, in Helmand Province.

Marines engaged in a firefight with the Taliban after the U.S. troops landed in helicopters near the town. They fired at least four rockets at fighters who attacked from compounds.

At least one Marine was wounded by shrapnel.

Civilians

The town, a network of desert irrigation canals constructed decades ago under a U.S.-funded development plan, is a place where the Taliban administers a government and controls a comparatively large population of 75,000 to 100,000 people.

More than two hours later, the area was still gripped by the firefight, with the Marines firing another large rocket. One family of civilians nearby was huddled in a room of their house, with the washing flapping on the line outside.

NATO forces have told civilians not to leave their homes, though it is uncertain how heavy the fighting will get.

"The international forces must adopt certain procedures and mechanism during operation in Marjah to protect civilians," Afghan President Hamid Karzai said in a statement.

The offensive began with waves of helicopters ferrying U.S. Marines into the city in the early morning hours. British troops then flew into the northern part of the surrounding Nad Ali district, followed by tanks and combat engineering units.

"We have seized 11 key locations in the district and the resistance from the insurgents has been subdued," Helmand Governor Gulab Mangal told a news conference.

Thousands of civilians have been killed in U.S.-led foreign troops' attack since U.S: invasion in Afghanistan, U.N. said.

15,000 Troops in operation 

The 15,000-troop operation was named Mushtarak, or together, perhaps to highlight that NATO and Afghan forces are determined to work within the assault.

U.S. Marines called in two Harrier jets which flew over a Taliban position and unleashed machinegun fire.

"We are currently moving to seize our objective. We have been in contact for five hours from the southwest, north and east and we are moving to push to finish securing the areas of insurgents still," Lt. Mark Greenlief told Reuters.

Marjah has long been a breeding ground for fighters and lucrative opium poppy cultivation. Residents may not be keen for any upheaval.

Isaf and the Afghan government have stressed that they hope civilian casualties will be avoided, publicising their operation in advance.

Hundreds of civilians fled the area, but many have stayed.

Jamil Karzai, the head of the Afghan government's commission for national security, and a relative of the Afghan president, warned that publicising the operation so heavily in advance had given away military advantage.

Agencies