Thousands of U.S. and Afghan troops began a major offensive on Saturday by moving into the Taliban stronghold of Marjah, but their progress was slowed by a dense thicket of strategically placed bombs and booby traps.

The air and ground offensive is largest since the Afghan war began in 2001. It's aimed at establishing the authority of the Afghan government in the region.

Marjah, in southern Helmand province, is the largest town under militant control. The long-anticipated attack aims to break the Taliban's control of a wide area of the southern heartland.

Maj. Gen. Nick Carter, NATO commander of forces in southern Afghanistan, said Afghan and coalition troops made a "successful insertion" into Marjah. He said the operation was going "without a hitch."

The attack was bolstered by action further north. Thousands of British, U.S. and Canadian troops moved into the Taliban areas to Marjah's north to clear out villages that have been under Taliban control for years.

At least 20 insurgents were reported killed in the Helmand operation, said Gen. Sher Mohammad Zazai, the commander of Afghan forces in the region. Kalashnikov rifles, heavy machine-guns and grenades have been seized from the 11 insurgents captured so far.

An American and a British soldier were killed during the first day of the offensive, according to military officials in those countries. Three more U.S. soldiers died in a bombing elsewhere in southern Afghanistan Saturday.

The long-anticipated attack on Marjah is the largest since the 2001 U.S.-led invasion of Afghanistan. It is also the first attack since U.S. President Barack Obama sent 30,000 American reinforcements to Afghanistan in December to try to reverse the direction of the war.

The attack began with more than 30 transport helicopters carrying NATO troops into the centre of Marjah before dawn Saturday, as British, Afghan and U.S. troops spread out across the Nad Ali district to the north.

The operation was well-publicized, in an attempt to clear civilians of the area, CTV's Janis Mackey Frayer told Â鶹´«Ã½ Channel Saturday.

Publicizing the attack is part of a new NATO strategy aimed at protecting civilians.

Afghan President Hamid Karzai called on coalition troops to "exercise absolute caution to avoid harming civilians." In a statement, he also asked insurgents to give up violence and return to civilian life.

Taliban claim control of region

A Taliban spokesperson insisted that militants were maintaining their hold of Marjah and resisting the coalition attack.

"The Taliban are there and they are fighting. All of Marjah is still under Taliban control," Qari Yousef Ahmadi told The Associated Press. He would not say how many Taliban fighters remained in Marjah but dismissed NATO reports on the battle as "propaganda."

U.S. Marines involved in the attack told The Associated Press that Taliban posts on the front lines of town seemed to have been abandoned, an elaborate network of explosives and booby traps left in their wake proving the biggest obstacle for troops on the ground.

Lt. Col. Brian Christmas, commander of the 3rd Battalion, 6th Marines, said U.S. troops faced sustained gunbattles in four areas of Marjah.

After the Marines worked slowly through a suspected minefield and cleared civilian compounds of bombs, their Afghan counterparts interviewed civilians.

Shopkeeper Abdul Kader, 44, said seven or eight Taliban fighters had fled in the middle of the night. They had been holding the position where the Marines crossed over a bridge.

Kader expressed anger at the insurgents for planting bombs and mines around his neighbourhood.

"They left with their motorcycles and their guns. They went deeper into town," he said, as Marines and Afghan troops searched a poppy field next to his house. "We can't even walk out of our own houses."

Operation Moshtarak -- which means "together" in Dari -- involves more than 15,000 troops. The government says Afghan soldiers make up at least half of the offensive's forces.

There are about 30 Canadian mentors on the ground with an Afghan battalion, Mackey Frayer said. "This is being seen as a test of the competence of the Afghan forces,.

Three Canadian CH147 Chinook helicopters, backed by four tactical Griffons, were part of a wave of aircraft that launched the Marjah attack under darkened dawn skies, carrying troops to the area.

After securing Marjah, a town of about 80,000 people about 610 kilometres southwest of Kabul, NATO hopes to bring aid and public services to the region in Helmand province.

Returning those services will be key to the Afghan government's success in restoring control of the region and keeping the Taliban from returning.

International development workers and Afghan officials are waiting to enter the area, once coalition troops have made it sufficiently safe. Government officials have already selected locations where they plan to build schools, clinics and mosques.

Carter said coalition forces hope to have an Afghan government presence in the region within the next few days.

As part of that effort, a series of meetings are to be held with tribal elders in the area, said NATO's civilian chief in Kabul, Mark Sedwill.

"I can't yet say how long it will take for this military phase to get to the point where we can bring in the civilian support from the Afghan government. We hope that will happen quickly," Sedwill said.

With files from The Associated Press and a report from CTV's Janis Mackey Frayer in Afghanistan