CAIRO, Egypt - Osama bin Laden's deputy urged Somalia's Islamic militants to remain steadfast in their battle against "invaders" and crusaders" in an audiotape posted on the Internet Friday.

The militants have been driven out of the capital Mogadishu and much of the southern part of the country by Ethiopian-backed government forces after more than six months in power. Many have retreated to the southern tip of Somalia and vowed to keep fighting, raising the specter of an Iraq-style guerrilla war.

"I speak to you today as the crusader invader forces of Ethiopia violate the soil of the beloved Muslim Somalia," Ayman al-Zawahri, al-Qaida's No. 2, said in the audiotape.

"I call upon the Muslim nation in Somalia to remain in the new battlefield that is one of the crusader battlefields that are being launched by America and its allies and the United Nations against Islam and Muslims," al-Zawahri added.

Ethiopia is a U.S. ally and American Navy forces are deployed off the Somali coast to prevent the militants from fleeing by sea. The Somali government is supported by the United Nations.

The authenticity of the audiotape, which lasts more than five minutes, could not immediately be verified. But the message was aired on a Web site frequently used by Muslim militants and carried the logo of al-Qaida's media production wing, al-Sahab.

The audiotape is the first al-Zawahri has issued this year, having boosted his media messages in 2006.