Somalia's Al-Qaeda-linked Shebab militants have appointed a successor to their former leader who died in a US airstrike, the private terrorism monitoring group SITE said Saturday.

In a statement posted on jihadi forums, the rebels announced the appointment of Ahmad Umar to replace Ahmed Abdi Godane and vowed revenge for his killing, SITE said.

Word of the appointment came shortly after the rebels confirmed to AFP that Godane had been killed by a US drone on Monday in their first official comment on his fate.

"Avenging the death of our scholars and leaders is a binding obligation on our shoulders that we will never relinquish nor forget no matter how long it takes," SITE quoted the Shebab statement as saying.

"By the permission of Allah, you will surely taste the bitter consequences of your actions ..."

The group also renewed its pledge of allegiance to Al-Qaeda chief Ayman al-Zawahiri, according to SITE.

Throughout the week, the rebels had refused to confirm or deny reports of Godane's death.

On Friday the Pentagon confirmed Godane died in an attack in which US drones and manned aircraft rained Hellfire missiles and laser-guided bombs on a gathering of Shebab commanders.

US officials did not specify how his death was confirmed. But in similar cases in the past, US intelligence agencies have tested DNA samples and used information gleaned from eavesdropping.