Afghan security forces arrive to fight with Taliban forces in Helmand, Afghanistan October 21, 2015. REUTERS/Stringer/Files

Afghan government forces have pulled out of a second district in Helmand, officials said on Monday. The Taliban now control most of the northern part of the province after troops withdrew from Musa Qala district last week.

The withdrawals cast doubt on the capability of the Afghan security forces to defeat the Taliban since the departure 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 is adjacent to Musa Qala, aiming to focus their strength on defending the area surrounding the provincial capital Lashkar Gah and the main highway between Kabul and the western city of Herat.

U.S. officials said that the Taliban group already dominates or threatens around a third of Afghanistan despite their continued failure in taking 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 enforce extremist 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 fought to conquer the Taliban, has been falling out of government control for months as the rebels choke much of the countryside outside a few district centers.

After the latest move, security forces are clinging to the town of Sangin, north of the main Highway One in addition to several 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“We have withdrawn our forces from Nawzad and Musa Qala based on military plans,” said Mohammad Rasoul Zazai, a spokesman for the 215th army Corps.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, while affirming the Afghani troops’ ability to take them back at any time.

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

Nonetheless, the surprising retreats leave the Taliban poised to advance toward 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 assist in training the Afghan army and hundreds more American troops were recently sent to bolster 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.