Local media have reported deadly clashes between protesters and Somaliland's security forces in Laascaanood, the capital of Sool region.
The Somali news website Hiiraan Online reported on Monday that at least twenty people had been killed over five days of protests.
Internet and phone lines have now been cut off by Somaliland's security forces in the city.
Laascaanood is near the border between the self-proclaimed republic of Somaliland and the Somali semi-autonomous region of Puntland.
The protesters want their city to be run by Puntland rather than Somaliland.
The UN Mission for Somalia condemned the violence in a statement released on Sunday.
"International partners are concerned about the violence in Laascaanood resulting in civilian deaths and casualties," it said.
"Preventing violence and ensuring the protection of civilians is paramount. We call for calm and restraint. Tensions must be resolved through de-escalation and dialogue," the statement added.
Protests began last Tuesday following the killing of a politician by gunmen.
"More than 10 people were killed in Laascaanood," Abdirahman Iro, head of the main opposition party, Wadani, had said at a rally on Saturday.
A large number of police were deployed in Las Anod on Saturday, and most businesses remained closed, residents told AFP.
"More than 30 people have been admitted to Las Anod hospital, including women and children, all of them with gunshot wounds and the town is still under tension," said Mohamed Suleyman, a resident of the town, had told AFP on Saturday.
Somaliland, a former British territory, declared its independence from Somalia in 1991, an act not recognised by the international community.
This region of 4.5 million people has remained poor and isolated ever since. But it has enjoyed relative stability as Somalia has been ravaged by decades of civil war and Islamist insurgency.