Roughly 15,000 American, Afghan and NATO forces began an assault late Friday on the Taliban in the central Helmand town of Marjah in what senior military commanders are calling the largest operation since the start of the Afghanistan war.

Punching their way through a line of insurgent defenses that included mines and homemade bombs, ground forces crossed a major canal Saturday into the town's northern entrance.

Maj. Gen. Nick Carter, NATO commander of forces in southern Afghanistan, said Afghan and coalition troops, aided by 60 helicopters, made a "successful insertion" into Marjah without incurring any casualties.

"The operation went without a single hitch," Carter said at a briefing in the provincial capital of Lashkar Gah.

Carter said the strike force quickly gained ground as it moved into Marjah and overran disorganized insurgents. "We've caught the insurgents on the hoof, and they're completely dislocated," he said.

At least 20 insurgents have been killed and 11 arrested so far in the offensive, said Gen. Sher Mohammad Zazai, the commander of Afghan forces in the region. Troops have recovered Kalashnikov rifles, heavy machine guns and grenades from those captured, he said.

The U.S. military announced two NATO troops were killed, the first reported coalition casualties of the offensive. A NATO statement said one service member died in an IED strike, while another died from small-arms fire. It did not give their nationalities.

U.S. military spokesman Lt. Col. Todd Vician said the deaths occurred in Helmand province and were related to the ongoing offensive aimed at breaking the Taliban grip over a wide area of their southern heartland.

The troops' advance into Marjah was slowed during the morning as they carefully picked their way through poppy fields lined with homemade explosives and other land mines.

Gunfire was ringing through the town by midday Saturday. The bridge over the canal into Marjah from the north was so rigged with explosives that Marines erected temporary bridges to cross into the town.

Lance Corp. Ivan Meza, 19, was the first to walk across one of the flimsy bridges.

"I did get an adrenaline rush, and that bridge is wobbly," said Meza, a Marine combat engineer from Pismo Beach, California, who is with the 1st Platoon, Lima Company, 3rd Battalion, 6th Marines.

Several civilians hesitantly crept out of compounds as the Marines slowly worked through a suspected mine field. The Marines entered compounds first to make sure they were clear of bombs, then called in their Afghan counterparts to interview civilians inside.

Shopkeeper Abdul Kader, 44, said seven or eight Taliban fighters who had been holding the position where the Marines crossed over had fled in the middle of the night. He said he was angry at the insurgents for having planted bombs and mines all around his neighborhood.

"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."

The ground assault followed many hours after an initial wave of helicopters carrying hundreds of U.S. Marines and Afghan troops swooped into town under the cover of darkness early Saturday. Cobra helicopters fired Hellfire missiles at tunnels, bunkers and other defensive positions.

Officials in the area have been signaling the offensive for weeks with hopes that innocent civilians and dispassionate Taliban fighters would flee the area. Commanders are stressing the importance of protecting the population rather than simply killing the resistance. Yet there have been reports civilians are having trouble getting out.

"They (Taliban) don't allow families to leave. Families can only leave the village when they are not seen while leaving," Qari Mohammad Nabi, a Marjah resident, said Friday shortly before the invasion.

The objective of Operation Moshtarak, which means "together" in Dari, is to secure the region from narcoterrorism and establish basic services under supervision of the Afghan government, a senior defense official told Fox News. To do that, Afghan and NATO forces need to defeat the remaining insurgents and drastically reduce the number of heroin-producing crops that fund the Taliban. U.S. commanders are confident they'll win the fight, but removing the poppy crops will prove harder than the battle.

Helmand's poppy harvest produces 60 percent of the world's opium, and Marjah has become a significant trade route for the drug. The area is filled with poppy-producing farmers and militants who organize and profit from heroin production. Locals complain the drug business is not only dangerous, but it fuels widespread corruption and distrust in government.

Afghan officials are hoping they can convince farmers to switch from growing poppy to wheat. NATO-led teams will provide farmers with seed and loans to sustain them during their transition. Farmers who aren't persuaded will have their crop chopped at the stalk by Afghan security forces.

U.S. military commanders say it's critical that the locals see this as an Afghan-led mission. Of the 15,000 troops involved in the Marjah offensive, roughly 5,000 are Afghan National Army units and 1,900 are Afghan National Police. The rest are mainly U.S. Marines, Army Strykers and other NATO forces.

Marjah is a town of around 80,000 people, and an estimated 1,000 Taliban are burrowed in to fight. However, the biggest threat comes not from the Taliban, but from the mines and improvised explosive devices they've had time to hide along the town's entry points.

"This may be the largest IED threat and largest minefield that NATO has ever faced," said Brig. Gen. Larry Nicholson, commander of Marines in southern Afghanistan.

The operation is seen as a major test for the Afghan government and for President Obama's strategy to surge troops into the country. If it goes according to plan, the Taliban will lose critical source of funding, the Afghan government will gain legitimacy and Obama can claim his first real victory in a war many believe the U.S. is losing.

The Associated Press contributed to this report.