Somalia's Islamist militant group of Al-Shabaab has joined ranks with al Qaida, the two sides announced the move in a joint video posting on the internet.

Ayman Al Zawahiri, leader of al Qaida Network said in a video posted online that he was breaking "the good news" that the Somali radical group of Al-Shabaab has joined global Jihadist movement, saying the move will "annoy the crusaders".

Leader of Somalia's Al-Shabaab, Ahmed Abdi Godane, known as Mukhtar Abu Zubair, who spoke in the first part of the 15-minute long video, said in his group's fighters will be "faithful soldiers" for the al Qaida leader.

"In the name of my mujahedeen brothers, leaders and soldiers... I pledge obedience," the Al-Shabaab leader told al Qaida's chief Zawahiri, according to translations quoted by world media.

The Somali militant group has in the past pledged allegiance to the former al Qaida leader Osama bin Laden who was killed in a U.S. covert raid at his home in Pakistan last year.

"Lead us on the road of jihad and martyrdom, in the footsteps that our martyr Osama bin Laden had drawn for us," said the Al Shabaab leader in the Arabic Language video addressing the current al Qaida head Al Zawahiri.

Al-Shabaab controls much of south and center of Somalia but has lately been facing mounting military pressure as allied Somali, Ethiopian and Kenyan troops as well as African Union peacekeeping forces launched military offensive.

The radical group has lost increasing ground to the allied regional forces since fleeing most parts of the Somali capital, Mogadishu, in August last year.

Both Ethiopian and Kenyan troops crossed into Somalia to chase the militant movement from their bases in the south and center of the Horn of Africa country.

The latest announcement by al Qaida and Al-Shabaab marks the first time the two sides have formally joined forces after being allies in their respect wars. It is not clear if the Somali group will keep its name of change it to reflect its new position as al Qaida outpost in Somalia and the Horn of Africa.