US Marines are in a "hell of a fight" as they storm into Taliban strongholds in a major assault in Afghanistan, their commanding officer said on Friday.

Nearly 4,000 Marines launched the operation Thursday in parts of the southern province of Helmand, suffering their first fatality in a pivotal test of President Barack Obama's aggressive new strategy against the Taliban.

The 1/5 Infantry Battalion met only light resistance in their push south and had already been able to meet locals at shuras (councils), Brigadier General Larry Nicholson said, speaking to a convoy with which AFP was travelling.

But "for 2/8 there is a hell of a fight going on in the southern quarter of the sector," the top Marine said on arrival at Garmsir, a town along the Helmand River that was a key objective for the offensive.

"2/8 are going to face some challenges," he said. The Marines were in an area called Toshtay about 25 kilome