African nations sending peacekeepers to Somalia should convince Islamists who control much of the country it is not an "invasion force", the UN chief says.
Both government and Islamist forces have been preparing for war
"It is important that we get the Somalis to understand that the force is coming in to help," said Kofi Annan.
The Islamists have said they will fight any peacekeepers sent to Somalia.
The UN Security Council has approved plans to send an 8,000-strong force to protect the weak government in its base in Baidoa, north of the capital.
"They are coming there to help them stabilise the situation, to help their people, and they are not coming in as an invasion force of any kind," Mr Annan said.
The US-led resolution, adopted unanimously by the 15-nation Security Council, said Somalia's transitional government represented "the only route to achieving peace and stability" in the country, which has been without effective central government since 1991.
The resolution also said the international arms embargo on Somalia should be eased to help the government re-arm.
Observers fear a regional conflict breaking out in Somalia, with rivals Ethiopia and Eritrea backing opposing sides.
Meanwhile, the Islamists say that Ethiopian forces who they accuse of deploying to support the government have shelled a town under their control.
Bandiradley, 630 kilometres (390 miles) north east of the capital, Mogadishu, is the most northern town taken by the Union of Islamic Courts (UIC) during its five-month advance.
"Ethiopian soldiers have massed around Bandiradley soon after the arms embargo had been lifted and started firing missiles toward our positions," said UIC official Sheikh Abdullahi Ali Hashi.
One pro-government fighter was reportedly killed in the clashes.
Ethiopia has repeatedly denied reports that its troops are fighting in Somalia but admits having hundreds of military trainers with the government.
Ethiopia and other states which border Somalia were excluded from contributing to the proposed African peace force.
The Islamists have declared a "Holy War" on Ethiopia, which has twice fought wars with Somalia.
Despite the exclusion of the "front-line states" the UIC still reject the idea of foreign peacekeepers, saying Islamic laws have already brought stability to areas they control.
They have held several rallies in Mogadishu against the proposed force.
"We see this as creating instability in Somalia. Most of Somalia is peaceful," Union of Islamic Courts official Ibrahim Adow told the BBC's Network Africa programme.
"Deploying foreign forces to Somalia is seen as invading forces and the Somali people are prepared to defend themselves against aggression," he said.
The Islamists have held protests against peacekeepers
However, observers believe it will be a long time before any peacekeepers arrive and say the UN resolution may be primarily intended to show symbolic support for the government.
The East African body, Igad, which is supposed to supply the troops is understood to be split over the idea.
Somalia's government has welcomed the resolution.
"We thank all the members of the Security Council, especially the American government," Deputy Defence Minister Salad Ali Jelle told Reuters news agency.
"This will bring solutions not war."
The government and the Union of Islamic Courts are due to hold peace talks in Sudan next week.
The US, Ethiopia and the Somali government also fear the Islamists are offering shelter to al-Qaeda operatives, a charge they deny.
A recent UN report accused several countries, including Ethiopia and Eritrea, of breaking the international arms embargo.