Fighting across Afghanistan has picked up in recent weeks with the return of warmer weather, putting government forces under pressure in several areas, underlining the risk to parliamentary and district council elections due in October.

Thursday night's loss of Kohistan, north of Fayzabad, the capital of Badakhshan, came after security forces failed to receive supplies or reinforcements and pulled out of district police headquarters, provincial police spokesman Sanaullah Rohani said.

A number of security posts in Teshkan district, to the south of Fayzabad, were also abandoned to the insurgents, who were stepping up pressure in the area, he said.

Taliban spokesman Zabihullah Mujahid said 15 members of the security forces had been killed and 14 wounded with three pickup trucks and a large number of weapons seized, while two Taliban fighters were killed.

Although Badakhshan, which shares a border with Tajikistan, China and Pakistan, is not one of the Taliban heartlands, the insurgents have been making gains there and the loss of Kohistan brings to three the number of districts they control in the province.

Mountainous and remote, the region contains rich mineral reserves and numerous unregistered gemstone mines which are exploited by local commanders and the Taliban, according to local officials.

Voter registration has been underway across Afghanistan but many people have been reluctant to sign up for fear of attacks on voter centres or retaliation by the Taliban, which is opposed to the elections.

US military estimates show the Western-backed government in Kabul controls areas with about 65 percent of the population.

In terms of districts, the government controls or influences 56.3 percent of the country, the second lowest level since at least 2015, the latest report from the Special Inspector General for Afghanistan, a US congressional watchdog, shows.