Heavy fighting has erupted between the forces of the self-declared Somaliland republic and those of Puntland, an autonomous region of Somalia.
This is the second time in a month that clashes have broken out near the disputed border town of Las Anod.
Correspondents say that Somaliland was worried by the election earlier this month of Puntland leader Abdullahi Yusuf as the new president of Somalia.
The fighting comes as donors meet in Sweden to discuss rebuilding Somalia.
The BBC's Mohamed Olad Hassan in the Somali capital, Mogadishu, says that at least nine people have been killed in the clashes, some 30km north of Las Anod.
Las Anod is the capital of Sool, which both regions claim, along with neighbouring Sanaag.
Our correspondent says this is the heaviest fighting between the two regions, featuring heavy artillery, rocket-propelled grenades and anti-aircraft missiles.
Much of Somalia is in ruins after years of fighting
Both sides have accused the other of starting the hostilities.
Mr Abdullahi was elected by a transitional parliament in neighbouring Kenya and plans to return to Somalia within the next two months.
The donor conference in Sweden is being attended by officials from the United Nations, the European Union, the Arab League and the African Union.
Somalia has been without a functioning national government since the overthrow of President Mohammed Siad Barre in 1991.
Rival warlords have battled for control of territory ever since and millions of Somalis have fled the country.