Somalia's federal troops and armed opposition groups clashed in heavy fighting in Baidoa city centre on Monday, a local resident and an army officer told Reuters. 

The administrative capital of South West state, Baidoa is home to ​international peacekeepers and humanitarian agencies. It is one of Somalia's largest cities with a population of over a million people.

Fresh fighting in the city could compound a dire humanitarian crisis in the area. Baidoa hosts hundreds of thousands of displaced people and one in four children at displacement sites there are severely malnourished, according to a July survey by Medecins Sans Frontieres.

"Militias loyal to the former Baidoa president entered Baidoa from two sides, challenging two battalions of federal military. Fighting has intensified now," Farah Nur, a shopkeeper in the city, told Reuters.

Federal troops have been in charge of the city since March when they seized control, which prompted South West state leader Abdiaziz Hassan Mohamed ​Laftagareen to resign.

Nur told Reuters federal forces were using heavy weapons and that he had seen at least two civilians killed in their houses by stray bullets and that several others who were injured were being taken to hospital.

Hassan Mohamed, ​Laftagareen's spokesperson, said in a post on Facebook that their troops had captured Baidoa and asked federal forces to surrender. His claim could not be independently verified.

"The armed militias attacked Baidoa this morning, we are pushing them back....there are casualties as stray bullets enter houses," Captain Osman Abdullahi, a Somalia federal military officer, told Reuters.