KANDAHAR, Afghanistan — Taliban fighters overran checkpoints in a nighttime raid in Afghanistan's volatile southern Helmand province, killing at least 20 police officers as the battle raged into Saturday, authorities said.

The assault came as Afghanistan's military acknowledged the Taliban controls at least four districts across the country.

The attacks in Helmand hit police checkpoints in the Musa Qala district, long a Taliban stronghold, said Mohammad Ismail Hotak, the head of the province's joint coordination of police and military operations.

He said the attacks wounded at least 10 officers, though the Taliban also seemed to have suffered high casualties.

Saqi Jan, the head of police logistics in Musa Qala, said area checkpoints were manned by officers from the neighboring district of Baghran who had been forced out by earlier Taliban attacks.

"Baghran has been under Taliban control since last year, so these police came to Musa Qala and built themselves a small compound and some checkpoints," he said.

The Taliban claimed responsibility for the attacks. The militants have been targeting vulnerable police checkpoints across the country since launching their summer offensive in April. Last month, a Taliban attack in Helmand's Naw Zad district killed at least 19 police officers.

Afghan army Gen. Afzel Aman, the head of the Defense Ministry's operational department, told journalists Saturday that four districts are now under Taliban control in the country: Nawa in Ghazni province, Baghran and Dishu in Helmand province and Khak-e Afghan in Zabul province.

"Fighting is going on almost everywhere compared to last year, and many places are under threat of enemy attack," Aman said.

Afghan forces are fighting alone this year as the U.S. and NATO have ended their combat mission and their casualties are soaring. Between Jan. 1 and May 7, 2,322 army, police and local police personnel were killed, 53 percent more than the same period in 2014, according to NATO.

By comparison, a total of 2,217 American service members have been killed since the 2001 invasion.

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Associated Press writer Rahim Faiez in Kabul, Afghanistan, contributed to this report.