KANDAHAR, Afghanistan — Culminating a deadly week for Afghan forces in the south of the country, an airstrike from friendly forces mistakenly struck an Afghan unit in Helmand Province on Sunday, killing at least 10 security personnel, officials said.

The latest Afghan losses in Helmand come on the back of heavy fighting and attacks in the neighboring provinces of Uruzgan, Kandahar and Farah, which left dozens of soldiers and police officers dead. The Afghan Defense Ministry says fighting is taking place in 20 of the country’s 34 provinces, with intense violence in eight of them.

Omar Zwak, a spokesman for the governor of Helmand, said 10 people had been killed when the airstrike struck an Afghan police unit. He said the strike was part of an operation to clear areas in the Gereshk district threatened by the Taliban and advance the government’s control beyond the provincial capital, Lashkar Gah. Other officials said the number of casualties, and which arm of the Afghan security forces they involved, was still being figured out.

Both the Afghan and American militaries use air power. “We are investigating which air force had carried out the strike,” Mr. Zwak said. Defense Ministry officials have told the local news media that Afghan aircraft were responsible.