A US drone attack has killed 22 Afghan soldiers held by Taliban militants in the southern Helmand province, while Taliban have overrun a strategic district elsewhere.

Provincial officials announced the fatalities on Saturday. Taliban also confirmed the death toll, saying the airstrike had killed three of the group's members in the Nad-e-Ali district on Thursday.

Helmand is a strategically important province for both the Afghan government and Taliban militants, who control or contest 10 of the 14 districts in the opium-rich province.

On Saturday, Taliban militants seized a strategic district in the eastern province of Paktia, from which they can surge towards several other provinces.

Officials said dozens of police and soldiers were killed as the militants captured the Jani Khel district after five days of siege.

Local governor Abdul Rahman Solamal said hundreds of militants attacked police check posts overnight, prompting security forces to flee the district.

Jani Khel sits at an intersection linking eight districts. It also connects Paktia with neighboring Khost province and Pakistan.

"If we do not retake it (Jani Khel) soon then Taliban can easily move from one province to another and can undermine security in at least three provinces," Solamal warned.

More than 20 soldiers and police were killed and another 20 wounded in the fighting overnight, while some 200 Taliban insurgents were killed, he said.

In a statement, Taliban spokesman Zabihullah Mujahid said large amounts of equipment had been captured, including armored vehicles, light and heavy weapons and ammunition.

Taliban have regrouped since the death of former leader Mullah Akhtar Mansour earlier in the year and are reported to be currently in control of more than 65 percent of the country.

Fierce fighting is currently ongoing against the militants across the country, notably in Helmand and around the northern city of Kunduz, which they briefly seized last year.

Late last month, the United Nations Assistance Mission in Afghanistan (UNAMA) said it had recorded 1,601 civilian deaths and 3,565 injuries in Afghanistan in the first six months of 2016.

The mission warned that civilian casualties had hit a record high this year, describing them as "alarming and shameful."