The Islamists who control much of southern Somalia have warned they will fight any peacekeepers sent there, after the UN approved a peace force.
Both government and Islamist forces have been preparing for war
"We see this as creating instability in Somalia. Most of Somalia is peaceful," Union of Islamic Courts official Ibrahim Adow told the BBC.
The UN resolution backs the sending of an 8,000-strong African force to support Somalia's weak government.
It also called for the easing of an arms ban to let the government re-arm.
"Deploying foreign forces to Somalia is seen as invading forces and the Somali people are prepared to defend themselves against aggression," Mr Adow told the BBC's Network Africa programme.
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.
But Mr Adow said supporting the government, which only controls the area around the town of Baidoa, would create more trouble and complications in Somalia.
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 UIC say they are capable of restoring security to the country, so there is no need for foreign peacekeepers.
They have organised several demonstrations against foreign forces being deployed.
They see a peacekeeping force as cover for Ethiopian troops being sent to help the government, even though the UN resolution rules out troops from countries which border Somalia, such as Ethiopia.
The UIC has declared "Holy War" on Ethiopia, which has twice fought wars with Somalia, over control of Ethiopia's large Somali-speaking region.
Ethiopia denies sending troops to help Somalia's government but admits to providing military trainers.
Outgoing US ambassador to the United Nations John Bolton said not intervening was not an option.
"The other option is that the instability we have seen in Somalia for over 15 years would spread to the region," he said.
Why does the UN want to go in now that the Islamists have done a better job than the UN would ever have done.
"I think the choice of doing nothing is really not a choice at all."
The aim of the force would be to protect transitional institutions located in Baidoa, some 250km (155 miles) north-west of the capital.
Washington also fears the Islamists are offering shelter to al-Qaeda operatives, a charge they deny.
Experts worry a spiralling conflict could spill over into other countries in the Horn of Africa.
The fear in particular is that Ethiopia and Eritrea will come into conflict because they support opposite sides and might see in Somalia another battleground in which to continue the intermittent war over their own border dispute.
A recent UN report accused several countries, including Ethiopia and Eritrea, of breaking the international arms embargo.
The Islamists have removed the check-points, at which gunmen used to extort money from commercial vehicles since taking control of the capital, Mogadishu, in June.
The Islamists have held protests against peacekeepers
In some areas, they have imposed strict Sharia law, such as public executions of murderers and floggings for drug dealers.
Some cinemas which show foreign films and football matches and radio stations which ban Western music have been closed but in other areas under UIC control, such activities have been allowed to continue.
They have also banned the popular stimulant, khat, used by many Somali gunmen.