A recording said to be of Al-Qaeda's second-in-command has urged Islamists to join a fight against the Somali government and its Ethiopian allies.
The voice was said to be that of Ayman al-Zawahiri
In the recording on a website used by al-Qaeda, the man said to be Ayman al-Zawahiri urges the use of suicide bombs and ambushes seen in Iraq.
He said the West was waging a crusader war against Islam in Somalia.
In Somalia, government forces backed by Ethiopian troops are fighting fleeing Islamists near Kenya's border.
The BBC's Karen Allen in Nairobi says there are fears that a protracted Iraq-style guerrilla war could be launched by the retreating Islamists.
Somalia's interim President, Abdullahi Yusuf, has called for a "speedy deployment" of international peacekeepers in his war-torn country.
The voice said to be Zawahiri, though not yet confirmed as being him, called upon Muslims around the world to join a jihad in Somalia.
"I appeal to the lions of Islam in Yemen, the state of faith and wisdom, I appeal to my brothers the lions of Islam in the Arab Peninsula, the cradle of conquests, and I also appeal to my brothers the lions of Islam in Egypt, Sudan, the Arab Maghreb, and everywhere in the Muslim world to rise up to aid their Muslim brethren in Somalia," he said.
He said this could be done "through offering sacrifices, money, opinion, and expertise so as to defeat the slaves of America that it sends to death on its behalf".
Zawahiri also accused the United Nations of plotting against Somalia.
"The Security Council is plotting to approve this invasion by issuing its resolution to dispatch international forces to Somalia and by its failure to issue a resolution that calls for the withdrawal of the Ethiopian forces from Somalia," he said.
The speaker urged the fighters to employ the tactics that have been used by insurgents fighting US-led forces in Afghanistan and Iraq.
"As the world's most powerful power in the world has been defeated in Afghanistan and Iraq by the believer groups, who will go to heaven, its slaves will be defeated, God willing, on the land of the muhjahid Muslim Somalia," he said.
"Use ambushes, mines, raids and martyrdom-seeking raids [suicide bombings] to devour them as the lions devour their prey."