Al-Qaeda's new leader Ayman al-Zawahri said he brought "glad tidings" of a union with al-Shabab. REUTERS/Social Media Website v

Osama bin Laden's long-time lieutenant Ayman al-Zawahri has taken over as the new Al Qaeda chief after the killing of the group's founder and leader, Osama Bin Laden, an Islamist website reported on Thursday.

Egyptian-born Zawahri had vowed earlier this month to press ahead with Al Qaeda's campaign against the United States and its allies, in what appeared to be his first public response to Bin Laden's death in a U.S. commando raid in Pakistan in May.

The general leadership of Al Qaeda group, after the completion of consultation, announces that Sheikh Dr Ayman Zawahri, may God give him success, has assumed responsibility for command of the group, Islamist website Ansar Al-Mujahideen (Followers of the Holy Warriors) said in a posted statement.

Zawahri had been seen as Bin Laden's most likely successor. His whereabouts are unknown.

Believed to be in his late 50s, Zawahri met Bin Laden in the mid-1980s when both were in Pakistan to support guerrillas fighting the Soviets in Afghanistan.

In a video message posted on the internet on June 8, Zawahri said Al Qaeda would continue to fight.

The Sheikh (bin Laden) has departed, may God have mercy on him, to his God as a martyr, and we must continue on his path of jihad to expel the invaders from the land of Muslims and to purify it from injustice, Zawahri said, according to a Reuters report.

Today, and thanks be to God, America is not facing an individual or a group ... but a rebelling nation which has awoken from its sleep in a jihadist renaissance challenging it wherever it is.