Four peacekeepers have been killed by a roadside bomb as their convoy travelled through the north of Somalia's capital, Mogadishu.

The four men were part of the African Union forces operating in the region. An AU spokesperson also reported that five others were injured in the explosion.

It is just two months since 1,600 Ugandan AU soldiers were deployed to the Somali capital, and the latest developments mark the deadliest attack to directly affect the peacekeepers since their arrival.

A new force of 8,000 AU soldiers has been proposed to take over security duties from the Ethiopian army. Ethiopia's troops have been in Mogadishu since December helping the government to tackle Islamist insurgents and clan militiamen.

The latest attack has come after the United Nation's Emergency Relief Co-ordinator ended his visit to Somalia early in light of renewed security concerns in the African country.

The UN worker, John Holmes' arrival in Mogadishu coincided with a number of explosions which killed four people, one of which was close to the UN compound in the city.

Holmes is the most-senior UN worker to visit the city in a decade and has urged the nation's government to allow humanitarian aid to be transported to its people.

The UN humanitarian agency released a statement saying: "Mr Holmes returned to Nairobi today (Saturday) and plans for a second day in Somalia were cancelled."

The UN coordinator has said that the African Union could not boost its peacekeeping forces in the capital until the government increases security.

Up to 1,600 people were killed in six weeks of clashes between Ethiopian government-backed troops and Islamist and clan fighters before the Ethiopian government declared it had gained victory over the insurgents.

An estimated 20 per cent of Mogadishu's 2 million residents fled the city when violence erupted earlier this year, the BBC has reported.

The latest attack also came only a day after Somalia's President Abdullahi Yusuf held talks with his Ugandan counterpart Yoweri Museveni in Kampala over the peace mission.

"We lost four and five were wounded. It was a roadside bomb and its intention was to hit the peacekeepers," Mr Ankunda told Reuters news agency.

He told the BBC that the wounded would be evacuated to Uganda for treatment.

On Tuesday, the US and the AU urged Ethiopia not to withdraw its troops from Somalia before peacekeepers are deployed to replace them.

However, US Africa envoy Jendayi Frazer has said it would probably be several months before the full peacekeeping force arrived.

Ethiopia's prime minister says he wants to withdraw all his troops, after they helped oust the Union of Islamic Courts.