Al-Qaeda leader Ayman al-Zawahiri has urged all Muslims to support Somalia's Islamist Al-Shabaab insurgents, the SIE monitoring website reported Tuesday.
The Shabaab, a group which has been battling the Western-backed government in Somalia and proclaims allegiance to Al-Qaeda, has in recent months suffered several major setbacks.
An offensive led by African Union troops has stripped the Shabaab of most of their former bastions, including the southern port of Kismayu, a key asset Kenyan forces wrested back just over a month ago.
"This is a flagrant Crusader invasion of the Muslim countries and it makes jihad obligatory on every Muslim who is capable in Somalia and its surroundings and in the rest of the world until the invaders are expelled," the SITE Intelligence Group quoted Zawahiri as saying.
"Don't worry about the numbers of the Crusaders and their equipment, for these are spoils of war that are brought to you by the help of Allah," Zawahiri said.
"Therefore, make them taste the fire of jihad and its heat. Pursue them with guerrilla warfare, ambushes and martyrdom-seekers."
The website said the Shabaab had also posted several messages on the Internet vowing to intensify attacks in Mogadishu.
Since the Shabaab abandoned fixed positions in Mogadishu last year, under pressure from African Union troops, the war-ravaged seaside capital has been slowly coming back to life.
The Shabaab once controlled up to 80 percent of southern and central Somalia and the recent African offensive dashed their hopes of taking power.
But analysts have warned the Al-Qaeda-allied group could do just as much damage by reverting to guerrilla tactics and carrying out targeted attacks.
On Saturday, two suicide bombers attacked a Mogadishu restaurant popular with former exiles, wounding several people.