Kenya issued an arrest warrant and requested the extradition of a British citizen on Tuesday over the murder of a 21-year-old woman near a UK army training camp more than a dozen years ago, which has strained relations between the countries.

Agnes Wanjiru was found in a septic tank at the Lion's Court Hotel in the Kenyan town of Nanyuki in 2012, having last been seen at the hotel with a group of British soldiers.

A Kenyan magistrate concluded in 2019 that she had been murdered by British troops, but until now no formal request had been made to extradite a suspect.

The delay in securing justice has sparked outrage in Kenya, with Wanjiru's family and rights groups arguing that the killers were being shielded by a defence cooperation agreement that complicates prosecution of British soldiers training in Kenya.

"We are grateful to see the Kenyan government has acted, although it has taken too long, and kept the family in darkness," Wanjiru's niece Esther Njoki told Reuters by telephone.

Wanjiru, single mother of a then-four-month-old baby, was beaten and stabbed, and probably still alive when she was thrown into the septic tank, a magistrate said in 2019 in an inquest report that ruled that British soldiers had killed her.

"Justice Alexander Muteti granted the (arrest warrant), ruling that murder is an extraditable offence and that there was probable cause to institute murder charges against the accused," Kenya's Director of Public Prosecutions (DPP) said on Tuesday.

It did not identify the suspect or say whether the suspect was a soldier.

Britain's Ministry of Defence said it remained committed to helping the family secure justice, but that it would not comment further as the case was subject to ongoing legal proceedings.