MARJAH, Afghanistan — Thousands of U.S. Marines and Afghan soldiers stormed the Taliban stronghold of Marjah by air and ground Saturday, meeting only scattered resistance but facing a daunting thicket of bombs and booby traps that slowed the allied advance through the town.

The massive offensive was aimed at establishing Afghan government authority over the biggest southern town under militant control and breaking the Taliban grip in 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. He said the operation was going "without a hitch."

Thousands of British, U.S. and Canadian troops also swept into Taliban areas to the north of Marjah, seeking to clear a wide swath of villages that had been under Taliban control for several years.

No coalition casualties had been reported more than 12 hours after the initial airborne assault, but NATO said three U.S. soldiers were killed Saturday in a bombing elsewhere in southern Afghanistan.

At least 20 insurgents were reported killed in the Helmand operation. Troops have recovered Kalashnikov rifles, heavy machine guns and grenades from 11 insurgents captured so far.