"At least 20 people have been killed, and an estimated 119 others injured in Las Anod -- among them children -- in fighting between the security forces and members of a local clan," UN High Commissioner for Human Rights Volker Turk said in a statement.

"I am concerned by reports that the clashes are continuing today with additional claims of new casualties," he said.

On Monday, local elders had put the death toll at over 10, after angry demonstrations broke out in the disputed border town last month, with opposition parties and rights groups accusing government forces of shooting dead several protesters.

Las Anod is claimed by both Puntland, a northern state in Somalia, and Somaliland, which broke away from the rest of the country in 1991 but has not been recognised internationally.

The latest violence occurred hours after elders from the Sool region, where Las Anod is located, issued a statement pledging support for "the unity and integrity of the Somali Federal Republic" and urging Somaliland authorities to withdraw their forces from the area.

"These potentially unlawful killings come just a month after at least 20,000 people were displaced by clashes in Las Anod, and could contribute to further displacement, compounding the already fragile humanitarian situation in the region," Turk said.

The UN rights chief called on authorities to conduct a "credible and impartial investigation" into the clashes.

The aim, he said, should be to "determine who is responsible and hold them to account in fair trials, including for reported damage to homes."