Hundreds more US troops are headed for Afghanistan's strife-torn Helmand province to shore up security forces who have struggled in the face of sustained Taliban attacks, officials said.

The core of the new force will provide more security and act as advisers to the Afghan army's 215th Corps, US Army spokesman Col. Michael Lawhorn said in a statement.

Security forces in the southern province have been plagued by high desertion and casualty rates, corruption, and leadership problems, and the army corps recently saw more than 90 general officers replaced in a major shakeup.

"This was a planned deployment of additional personnel to both bolster force protection for the current staff of advisers and to provide additional advisers to help with ongoing efforts to re-man, re-equip, and re-train the 215th Corps," he said.

Officials have previously said the new troops would number roughly 200, but Lawhorn declined to publicise exact numbers, saying the reinforcements would be "significant".

The Nato-led coalition in Afghanistan declared its combat mission over at the end of 2014, and Lawhorn said the new troops in Helmand would be there "to train, advise, and assist our Afghan counterparts, and not to participate in combat operations".

Regular military advising is largely limited to the corps level and above, but coalition special operation advisers are still embedding at the tactical level with Afghan commandos, sometimes blurring the lines between advising and fighting.

American Special Forces advisers on the ground in Helmand have found themselves increasingly drawn into combat, with one Green Beret killed in January during a heavy firefight with Taliban insurgents. US warplanes conducted 12 air strikes during that fight.

Roughly 9,800 US troops remain in Afghanistan, but President Barack Obama's initial plan to withdraw forces by 2017 has already been scrapped, and top commanders are calling for an increased presence for at least five more years.

Helmand was one of the deadliest provinces for thousands of mostly British and American troops who fought there for more than a decade after a US-led military intervention toppled the Taliban in 2001.

After the coalition reduced its troop strength and transitioned to the focus on advising last year, it adopted a policy of "expeditionary advising" in Helmand, in which most foreign soldiers were not based permanently in the province, but flew in as needed.

That decision to withdraw permanent forces from Helmand was driven by the closure of bases and the reduction in the total number of foreign troops, US military spokesman Brig. Gen. Wilson Shoffner said in an interview last week.

"Expeditionary advising ... allows you to tailor what you send down there, but one of the challenging aspects of it is that we don't have the infrastructure and the permanent basing," he said.