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.

The withdrawals raise questions over the capacity of the Afghan security forces to take on the Taliban since the withdrawal of international forces 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. officials, the Islamist Taliban already control or threaten around a third of Afghanistan although they have so far failed to take over any major provincial centers 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 harsh Islamic rule 15 years after they were ousted from power.

Helmand, a major center 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 centers.

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 centers 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"We have withdrawn our forces from Nawzad and Musa Qala based on military plans," said Mohammad Rasoul Zazai, a spokesman for the 215th army Corps.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 downplayed 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 neighboring Kandahar provinces.

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

NATO's Resolute Support mission in Kabul has not commented on the decision to pull the troops out of the two Helmand districts, referring questions to the Afghan defense ministry.