A missile-firing drone killed at least six in Afghanistan on Monday including a veteran militant believed to have defected to Islamic State (IS) from the Taliban, Afghan officials said.

The senior militant, former Guantanamo Bay detainee Mullah Abdul Rauf, was killed in violence-plagued Helmand, officials in the southern province said.

Provincial police chief Nabi Jan Mullahkhel said Rauf was traveling in a car when the drone attacked. The other casualties included his brother-in-law and four Pakistanis, Mullahkhel said.

The United States operates drones over Afghanistan and neighboring Pakistan.

While Helmand officials said six people had been killed, the US army said coalition forces used a "precision, guided munition" to kill eight people who were considered a threat.

"We are working to confirm the identities of those killed in the strike," said a spokesman, Colonel Brian Tribus. He declined to say if the missile was launched by a drone.

Rauf has been influential in Afghanistan's jihadi movement for well over a decade.

Media reports last month said he had begun recruiting for Islamic State, part of a push by the movement to gain traction beyond its stronghold in Iraq and Syria.

Afghanistan's main intelligence agency, the National Directorate of Security (NDS), said in a statement Rauf was in charge of IS in southwestern Afghanistan and he was killed just after mid-day in "a successful military operation".

Islamic State last month announced it was expanding into Khorasan, a term Islamists use to describe a region encompassing Afghanistan and Pakistan, and declared a former commander from the Pakistani wing of theTaliban "governor" of the region.

Although there is little evidence of operational ties between IS and the Taliban, a number of militants have pledged allegiance to IS, apparently drawn by its successes in the Middle East.

Helmand's deputy governor, Mohammad Jan Rasulyar, said Rauf's membership of IS could not be confirmed but his associates were dressed in black outfits often worn by IS members.

A US military report released by the anti-secrecy group WikiLeaks in 2011 said that Rauf had tried to pass himself off as a low level Taliban worker who "delivered bread," but that interrogators suspected he was more senior.

Also in 2011, Newsweek magazine reported that Rauf had once led an elite fighting force close to Taliban leaderMullah Omar, and that after his return to Afghanistan in 2007 he had become the Taliban's shadow governor in Uruzgan province.

Guantanamo interrogators said Rauf had revealed detailed knowledge of Afghanistan's opium trade.