DUBAI (Reuters) - Al Qaeda leader Ayman al-Zawahri, in a statement on an Islamist website, urged Somalia's al Shabaab insurgents to fight back against what he called "crusader invaders" after Kenyan forces forced them out of their last urban bastion.

Somalia unveiled a new cabinet this week as it tries in a U.N.-backed initiative to restore central government control after 20 years of armed anarchy that has killed tens of thousands of people.

Backed by African Union (AU) peacekeepers, government forces have pushed the al Qaeda-linked al Shabaab insurgents out of the main urban areas they took over in their five-year-old revolt, spurring many Somalis to return to rebuild their country. But the capital Mogadishu remains vulnerable to militant attacks.

Zawahri singled out Kenyan forces that in September drove al Shabaab out of its last stronghold, the port of Kismayu, telling the militants not be worried by the large number of what he said was U.S.-backed African troops in Somalia.

"Show them the fire of jihad (Islamic holy struggle) and its heat. Chase them with guerrilla warfare, ambushes, martyrdom (suicide attacks)," Zawahri said in the video posted on Tuesday on a website used by Islamists.

He said AU forces were bound to be defeated just as U.S. forces had been forced to leave Iraq and Afghanistan and their missions in Libya, Yemen and Egypt had been attacked in a wave of anti-American violence across the Arab world in September.

"With God's grace, these people are to be defeated. They have been defeated in Iraq, they are withdrawing from Afghanistan, their ambassador was killed in Benghazi and their flags lowered in Cairo and Sanaa," he added. He was referring to demonstrators who attacked U.S. missions over an anti-Islam film produced in the United States.

It was not immediately possible to verify the authenticity of the recordings. But Zawahri, who took over after al Qaeda founder Osama bin Laden was found and killed by U.S. special forces last year, has regularly spoken on various countries in audio or video recordings posted on Islamist websites.

Zawahri also urged Somalis to back al Shabaab against Kenyan troops that entered Kismayu in south Somalia in September.

Al Shabaab still hold sway over sweeping rural areas of the Horn of Africa country where the Mogadishu government and regional administrations have minimal control.

Somalia inaugurated a new president in September, elected in the first vote of its kind since warlords toppled military dictator Mohamed Siad Barre in 1991, leaving the African nation without an effective central government.

(Reporting by Ahmed Tolba; Writing by Sami Aboudi; Editing by Mark Heinrich)