Over two weeks of clashes between rival militiamen in central Somalia have left at least 20 people dead, 120 wounded and forced some 90,000 from their homes, the UN says.

"Relative calm has returned... armed forces from both states have started to withdraw from the frontline," the United Nations Office for the Coordination of Humanitarian Affairs (OCHA) said in a statement on Tuesday.

The statement warned that the clashes had affected many people already struggling and in need of aid.

"The armed violence has exacerbated an already fragile humanitarian situation," the UN said.

Fighting broke out in the town of Galkayo on November 22 between gunmen loyal to the Puntland and Galmudug districts. Clashes continued after a ceasefire agreement last week, but the situation has been quieter.

Galkayo, which is located between the two districts, has been the scene of frequent fighting between rival political groups or clans, separate from the al-Shabab militants who are fighting the internationally-backed central government in Mogadishu.