Colonel Dave Butler, a spokesman for US Forces Afghanistan, said Afghan security forces had called for US air support during a firefight late Thursday in Lashkar Gah, in the southern province of Helmand.

The two parties had supposedly "deconflicted" their forces -- a military term for making sure neither side had troops in the strike zone.

The Afghan forces "confirmed the areas were clear of friendly forces", Butler said.

"Unfortunately, they were not and a tragic accident resulted. Afghan security forces as well as Taliban fighters were killed in the strikes."

Provincial government spokesman Omar Zwak told AFP that eight police officers had been killed in the strike, with 12 more wounded.

The death toll was confirmed by interior ministry spokesman Nasrat Rahimi on Facebook. He said 11 more police had been wounded.

A joint delegation had been appointed to investigate the incident, Rahimi said.

A third Afghan official -- head of the Helmand provincial council Ataullah Afghan -- gave a higher toll, saying 18 Afghan police were killed and 14 others were wounded.

Butler said the US and Afghan security forces were examining the "miscommunication" to ensure it didn't happen again.

"We regret this tragic loss of life of our partners and are committed to improvement every day with every mission," Butler said.

Afghan and US forces have intensified the aerial bombardment of Taliban and Islamic State group militants in recent months even as the US pushes for a peace deal with the insurgents after nearly 18 years of conflict.

According to US Air Force Central Command, the US dropped 7,362 bombs in Afghanistan in 2018, the highest number since at least 2010.

Friendly fire incidents are not unheard of in Afghanistan, and have bred deep mistrust between local and foreign forces.

In one of the deadliest, 16 Afghan policemen were killed in 2017 when they were mistakenly targeted by US air strikes in Helmand.

Civilian deaths from air strikes have also been increasing.

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