MARJAH, Afghanistan — Thousands of U.S. Marines and Afghan soldiers stormed the Taliban stronghold of Marjah before dawn Saturday, sweeping by air and ground against scattered resistance into the biggest southern town under militant control. The massive offensive was aimed at breaking the Taliban grip over a wide area of their southern heartland.

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 in southern Helmand province without incurring any casualties. He said the operation was going "without a hitch."

Thousands of British, U.S. and Canadian troops swept into Taliban areas to the north of Marjah. There have been no coalition casualties reported, but NATO said three U.S. soldiers were killed Saturday in a bombing elsewhere in southern Afghanistan.

At least 20 insurgents have been killed in the Helmand operation, said Gen. Sher Mohammad Zazai, the commander of Afghan forces in the region. In Kabul, Defense Minister Rahim Wardak told reporters at midafternoon that most of the resistance was centered around the main market district of Marjah.

The ground advance into Marjah was slowed by extensive fields of mines, homemade bombs and booby traps as Marine infantry crossed a major canal into the town's northern entrance. The town's canals were built by the U.S. in the 1950s and 1960s.

The few civilians who ventured out to talk to the Marines said teams of Taliban fighters were falling back deeper into the town, perhaps to try to regroup and mount harassment attacks.

The long-awaited assault on Marjah is the biggest offensive since the 2001 U.S.-led invasion of Afghanistan and is a major test of a new NATO strategy focused on protecting civilians. The attack is also the first major combat operation since President Obama ordered 30,000 U.S. reinforcements here in December to try to turn the tide of the war.