An adolescent boy and a young girl have been beheaded in two separate incidents in Afghanistan, local officials and police said on Friday, in the latest brazen attacks that have raised fresh questions about a splintering Taliban.

A 12-year-old boy was kidnapped and killed in southern Kandahar province on Wednesday, his severed head placed near his body to send a warning to police, said provincial governor spokesman Jawid Faisal.

The brother of the boy, neither of whom were named by officials, was a member of the Afghan Local Police (ALP), a US-trained militia charged with making Afghans in Taliban strongholds, like Kandahar, feel more secure, Faisal said.

"It's a Taliban warning to the ALP and to others who support the government," Faisal said of the killing, which happened in Kandahar's Panjwai district.

Taliban spokesman Qari Yousuf denied the group was involved.

Separately, a 6-year-old girl was beheaded in eastern Kapisa province on Thursday, said provincial police chief Abdul Hamed.

"We are not sure if she was beheaded by her family or the Taliban, but we know the Taliban control the area," Hamed said of the killing in Jalukhil village. He added that he could not send investigators to the area out of fears for their safety.

The murders follow the shooting or beheading of 17 young revelers attending a party in southern Helmand province this week, which officials said was the work of the Taliban, a charge the group also denied.

That massacre raised fresh concerns about Taliban leaders' grip on their scattered fighters, amid on-again, off-again peace moves by the group with the Afghan government.

"What we're seeing could be a new tactic by the Taliban to behead civilians to intimidate the population," said Faisal.

In Kandahar's Zhari district, officials also said on Friday that a 16-year-old boy accused by the Taliban of spying for the government was beheaded and skinned in late July.

NATO will withdraw most of its combat troops by the end of 2014, leaving Afghan forces in the lead security role.