MOGDISHU - Heavily armed Islamist militia who control the capital are advancing towards the seat of Somalia's interim administration, stoking fears of conflict and leading the government to put its troops on alert.

"We see it as aggression towards government-controlled areas and the people who support us," Prime Minister Ali Mohamed Gedi said from the government's provincial base in Baidoa.

Somalia has had no central rule since the 1991 ouster of a military dictator, and the Western-backed interim government's hopes of moving to Mogadishu were frustrated when the Islamists seized the capital last month from US-backed warlords.

The militia move to the town of Buur Hakaba on Wednesday inflamed the already high tension with the government, which many Somalis fear could boil over into a war for supremacy in the Horn of Africa nation of 10 million.

Islamist militiamen, riding on heavily armed pickup trucks known as "technicals", arrived in Buur Hakaba in the morning, the closest they have come to Baidoa from Mogadishu.

Islamist officials said their militia went to the hilly town - 60km from Baidoa - to receive 150 government troops switching allegiance, then headed back with them on the road to Mogadishu late in the day.

Whether they had gone any way beyond Buur Hakaba, on the road to Baidoa, could not be independently confirmed.

Gedi said the Islamists had breached an agreement signed with the government last month to stop military campaigns: "We call upon the Islamic courts to stop fighting. We hope they will end those acts and take part in peace efforts in our country."

"There is nobody who can take over Somalia by force."

The government, which despite its international recognition has no real authority on the ground beyond Baidoa, said it had received reports the Islamists were 35km from them.

President Abdullahi Yusuf was not in Baidoa but in his home region and stronghold Puntland, government sources said.

The Islamists confirmed their move to Buur Hakaba.

"It's true that 150 government forces have joined the Islamic courts. I am with them and I'm bringing them to Mogadishu," Sheikh Mukhtar Robow, a senior Islamic official in charge of defence, said.

Residents in Buur Hakaba town said the militiamen arrived in a convoy of more than 20 technicals, Somalia's version of tanks.

An Islamist source in Mogadishu, who asked not to be named, insisted there were no plans to attack Baidoa.

Two days ago the Islamists opened a sharia court in the government-controlled Bakol and Bay area, where Baidoa is located. It was the first time they had set up a court in an area they do not yet control since taking Mogadishu.