By Katy Migiro

NAIROBI (Thomson Reuters Foundation) - More than 75,000 people have fled their homes and 18 have been killed during three weeks of clashes in Somalia, the United Nations said on Monday, warning that women and children sleeping in the open will suffer as the rainy season looms.

Clashes erupted on Saturday and Monday between forces loyal to the two semi-autonomous regions of Puntland and Galmudug in the town of Galkayo, it said.

"The fear that the conflict may last longer than anticipated is driving more people out of their homes," the U.N.'s Office for the Coordination of Humanitarian Affairs said in an update.

Most of the displaced are women, children and the elderly who were already living in camps and have now fled for a second time to the outskirts of town, it said.

"The onset of the rainy season is likely to affect the displaced especially those spending nights in the open," it said.

Somalis hope that the Dehr rains, which fall between October and January, will alleviate drought which has left five million people short of food.

Galkayo, which is the capital of the north-central Mudug region of Somalia, is divided between clan militias loyal to the two regions.

The federal government is working to restore peace, the U.N. said.

Somalia has been at civil war for 25 years and clashes between the clan-based militias who control much of the country are common. In the south, forces loyal to the weak U.N.-backed government are also battling Islamist insurgents.

There are 1.1 million displaced people in Somalia, around one in ten people, often living in dire conditions.

(Reporting by Katy Migiro @katymigiro; Editing by Ros Russell; Please credit the Thomson Reuters Foundation, the charitable arm of Thomson Reuters, that covers humanitarian news, women’s rights, trafficking, property rights and climate change. Visit http://news.trust.org to see more stories.)