Two British soldiers are under investigation over the alleged murder of a Kenyan woman, it has been reported.

The Ministry of Defence said the matter was being investigated by the authorities in the East African country but would offer assistance if requested.

Agnes Wanjiru-Wanjiku disappeared from a hotel in Nanyuki on March 31. The 21-year-old's body was found on June 5 inside a septic tank on the hotel grounds, Sky News reported.

An MoD spokesman said: "This is an ongoing Kenyan investigation and as such it would be inappropriate for the Ministry of Defence to comment further. We stand by to offer any assistance and co-operation that is required."

Human rights organisation Social Watch wrote to the Kenyan department of public prosecutions complaining the case had not been properly investigated.

The organisation said witnesses reported Ms Wanjiru-Wanjiku was last seen with a British soldier.

The letter, obtained by Sky News, claims that "a night guard witnessed a fierce fight in the room Wanjiru and the British soldier were occupying".

The police were accused of taking a "casual stance" and of not being willing to pursue British personnel.

The department of public prosecutions has written to police in Nanyuki demanding a progress report on the case, Kenya's The Standard newspaper reported.

There is a British Army training base in the town, used to prepare troops for conditions in Afghanistan. In April this year it was reported more than 200 troops were involved in a brawl inside the Sportsman's Arms bar.