Police said the bodies of at least 13 counter-narcotics police officers had been recovered from the scene, in a remote district close to the border with Kandahar province.
"Counter-narcotics police were operating in Rig district in Helmand yesterday when there was an airstrike, which resulted in the deaths of a number of policeman," Omar Zwak, a spokesman for Helmand's governor, told Reuters.
US forces said they did not conduct air strikes in Helmand on Sunday. They did, however, say they bombed neighboring Kandahar province that same day. They declined to comment on whether the airstrikes had killed Afghan police, Reuters reported.
Helmand province is Afghanistan's major opium-producing center.
Zwak, the Afghan spokesman, said the incident was under investigation and it was too early to say how exactly how many had been killed.
As of Monday evening, 13 bodies had been taken to the provincial capital Lashkar Gah, but more could have been left at the site, local officials said, according to Reuters.
In July, eight Afghan soldiers were killed by a US airstrike in Logar province, south of the capital Kabul.