A mass grave containing the remains of 12 people has been uncovered in Afghanistan, Taliban officials said on Monday.

The grave was found by villagers over the past few days in the town of Spin Boldak, bordering Pakistan, a site of fierce fighting between former Afghan government forces and Taliban fighters before the hardline Islamists seized power last year.

Government spokesman Zabihullah Mujahid said the people were killed nine years ago when the US-backed government was in power but the site has not been independently investigated.

"These were individuals who were arrested from villages by the former cruel commander General Raziq. They were all civilians who were killed and buried in a mass grave," he said, referring to Kandahar's late police chief -- a powerful commander known for his effective battles against Taliban fighters during the 20-year war.

"We are looking into the issue of this mass grave after which we will decide on what kind of investigation should be conducted."

The remains have been reburied nearby, along with that of another individual found in a separate unmarked grave, said Haji Zaid, the spokesman for the governor of Kandahar province.

UN Special Rapporteur on human rights in Afghanistan, Richard Bennett, tweeted that it was "important these remains are not disturbed and damaged further pending forensic examination."

Videos posted to social media, shared by Taliban officials, show villagers gathered around heaps of bones.

General Abdul Raziq was shot dead by a bodyguard in October 2018, minutes after a meeting with then top US commander in Afghanistan, General Scott Miller.

The Taliban claimed responsibility, saying they had targeted Raziq who had a reputation of being a ruthless opponent in Kandahar and neighbouring provinces.

Raziq's brother Tadin Khan rejected the Taliban accusations.

"This is an attempt to malign our family," Khan, who had succeeded Raziq as Kandahar police chief, told AFP in a brief comment by telephone from the United Arab Emirates.

Human Rights Watch called for an investigation into the killings.

"Discovery of grave sites makes it all the more urgent the authorities prevent acts of revenge," tweeted Patricia Grossman, associate director at HRW.

The Taliban seized power on August 15 last year after government officials and army troops abandoned cities, but deadly clashes erupted between both sides in and around the town of Spin Boldak.

US and UK officials had even accused the Taliban of killing several former government officials and their relatives in the town after it fell to the hardline Islamists.