A senior Taliban official on Friday slammed Prince Harry after the royal disclosed he killed 25 people on military duty in Afghanistan and wrote it was like removing "chess pieces" from a board.

In his memoir to be released next week, Harry reveals the exact number of people he killed during two tours of duty, British media has reported.

"My number is 25. It's not a number that fills me with satisfaction, but nor does it embarrass me," he wrote in the book "Spare" due out Tuesday.

Anas Haqqani, a senior Taliban leader, criticised the Duke of Sussex over the remarks, saying those Harry killed were Afghans who had families.

"Mr Harry! The ones you killed were not chess pieces, they were humans," Haqqani tweeted, accusing the prince of committing "war crimes".

"The truth is what you've said; Our innocent people were chess pieces to your soldiers, military and political leaders.

"Still, you were defeated in that 'game'."

Afghan government spokesman Bilal Karimi also criticised Harry for his remarks.

"Such crimes are not limited to Harry, but every occupying country has a history of such crimes in our country," he tweeted.

"Afghans will never forget the crimes of the occupiers and will always keep the spirit of protecting their religion and country alive."

Harry served 10 years in the British military, rising to the rank of captain.

He carried out two tours of duty against the Taliban, first as a forward air controller calling in air strikes in 2007 and 2008, and later flying an attack helicopter in 2012 and 2013.

- 'Flashman braggadocio' -
 

Cameras mounted on the nose of his Apache chopper enabled him to assess his missions and determine with certainty how many he had killed.

He justified his actions using the memory of the 9/11 attacks in the United States and after meeting families of the victims.

Those responsible and their sympathisers were "enemies of humanity" and fighting them was an act of vengeance for a crime against humanity, he wrote in the book.

But his comments have come in for severe criticism in Britain.

Ben McBean, a fellow veteran who lost an arm and leg with the Royal Marines in Afghanistan in 2008 and who was described by Harry as a "real hero", slammed the prince for his remarks.

"Love you #Prince Harry but you need to shut up! Makes you wonder the people he's hanging around with," he tweeted.

Colonel Tim Collins, who became famous for a rousing speech he gave to his troops ahead of the war in Iraq, said the book was "a tragic money-making scam".

"Harry has now turned against the other family, the military, that once embraced him, having trashed his birth family," the retired commander told the Forces news website.

On Harry's claim about killing 25, Collins said: "That's not how you behave in the army; it's not how we think."

Referring to a fictional British soldier, journalist and royal author Tina Brown accused Harry of "boasting" with "Flashman braggadocio" about his exploits in Afghanistan.

"That's very unattractive. I don't think anybody is going to admire him for that in places that it counts," she told BBC radio.

It is not the first time Harry has sparked controversy regarding his Afghan service.

In 2013, he likened shooting insurgents to playing video games, prompting the Taliban to query his mental health.