LASHKAR GAH, Afghanistan (Reuters) - Afghan government forces have pulled out of a second district in Helmand, officials said on Monday, leaving the Taliban in control of most of the northern part of the province after troops withdrew from Musa Qala district last week.

Military and government officials said the move had been made to concentrate forces more effectively. But it raises questions over the ability of Afghan security forces to take on Taliban militants who have stepped up their insurgency since the withdrawal of international troops in 2014 from most combat operations left them fighting largely alone.

Army and government officials said security forces had left Nawzad district, which borders Musa Qala, and would concentrate their strength on defending the area around the provincial capital Lashkar Gah and the main highway between Kabul and the western city of Herat.

According to U.S. estimates, the Islamist Taliban control or threaten around a third of Afghanistan, although they have so far failed to take over any major provincial centres apart from their brief capture of the northern city of Kunduz last year.

The Taliban are seeking to topple the Western-backed government in Kabul and reimpose Islamic rule 15 years after they were ousted from power.

Helmand, a major centre of opium production where thousands of British and American soldiers and marines struggled to subdue the Taliban, has been slipping out of government control for months as the insurgents overrun much of the countryside outside a few district centres.

The latest move leaves security forces hanging on in the town of Sangin, north of the main Highway One, as well as a number of other towns and district centres including Gereshk, which lies on the highway and Marjah, close to Lashkar Gah.

“We have withdrawn our forces from Nawzad and Musa Qala based on military plans,” said Mohammad Rasoul Zazai, a spokesman for the 215th army Corps.

“Currently for us Sangin, Marjah, Nad Ali and surrounding areas of Lashkar Gah and Kabul-Herat highway are a priority. And we put all our efforts in these places,” he said.

Helmand governor Merza Khan Rahimi also played down the decision to withdraw from the two districts, which he said could be retaken at any time.

“It is normal during fighting to move forward or retreat,” he said. “We are not concerned about this.”

The surprise withdrawals nonetheless leave the Taliban poised to move on the nearby Kajaki district, the site of a huge hydroelectric dam built with millions of dollars of U.S. aid as part of a drive to provide power to Helmand and neighbouring Kandahar provinces.

U.S. Special Forces units have been in the region to help train the Afghan army and hundreds more American troops were sent to reinforce security for the training mission.

U.S. Army Col. Michael Lawhorn, a spokesman for NATO’s Resolute Support mission in Kabul, said the Afghan army was “making a tactical decision to reposition forces to fight the Taliban more effectively”. He added that the move would allow troops to be concentrated around fewer checkpoints, resulting in a more mobile force.