A South African soldier, who was part of a United Nations peacekeeping force in eastern Democratic Republic of Congo, died on Monday when a hand grenade exploded close to his sleeping quarters, South Africa's military said.

It was not clear what caused the hand grenade to explode, the South African National Defence Force (SANDF) said in a statement late on Tuesday, adding that a board of inquiry including UN officials would investigate the incident that happened in Beni.

South African soldiers are in Congo as part of the UN force called MONUSCO and a separate mission deployed by southern African regional bloc SADC to help the government fight rebel groups.

Including the latest death, at least seven South African soldiers have lost their lives in Congo since February.

Two were killed in a mortar attack on an SANDF base in Sake in eastern Congo last month, one was killed in a battle with Rwandan-backed M23 rebels in May.

Another died after receiving treatment for unspecified health issues in April and two were killed in a mortar attack in February.

At the time of the February deaths South African President Cyril Ramaphosa hit back at opposition criticism that the troops in Congo were ill-equipped.

The UN force is withdrawing from Congo in stages. A first stage was completed last month, but no timeline has been set yet for the next phase amid an escalation in clashes in the restive east.