Six years after thousands of US Marines flooded Afghanistan's southern Helmand province — an insurgent stronghold and a hub for Afghan drug production — American troops have returned to the volatile area to help counter yet another Taliban offensive. And already, one US soldier has been killed and another wounded.

The US military said on Tuesday that an improvised explosive device (IED) detonated near a joint patrol by US and Afghan forces in the area. Six Afghan soldiers were also wounded in a separate blast. The wounded American is in stable condition, the Pentagon said. The dead US soldier has not been identified, pending notification of his or her next of kin.

Around 100 US soldiers were deployed to Helmand over the past week after the Taliban offensive threatened the provincial capital Lashkar Gah.

"This is a big effort by the Taliban," Brigadier General Charles Cleveland, spokesperson for the US-led coalition forces in Afghanistan, told reporters on Monday. "This is probably the most serious push we've seen of the season."

The US had already been launching airstrikes against the militants, but Cleveland said the new forces will be strictly limited to advising Afghan troops and police in their fight against the Taliban.

"They're not about to go out and conduct operations or something like that," he said.

The Pentagon has not elaborated on what kind of operation the two US casualties were conducting when the IED explosion occurred, except to say they were participating in "train, advise, assist activities" with Afghan forces. In recent months, US ground troops have helped coordinate attacks by American aircraft during Afghan military operations.

US Marines helped the Afghan government establish control over Helmand in 2010, but the campaign came at the cost of thousands of US, coalition, and Afghan casualties. Foreign troops withdrew from the province in 2014, and the Taliban has since returned.

The soldier killed on Tuesday was the first US military fatality in Afghanistan since January. The US currently has around 9,800 troops in the Afghanistan.