Al-Shabab, the Somali armed group, has named Ahmed Umar, also known as Abu Ubaidah, as successor to Ahmed Abdi Godane who was killed in US airstrikes last Tuesday.

The armed group on Saturday pledged to take revenge for the death of one its founding leaders.

"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," the group said in a statement sent to Al Jazeera.

Omar, the group's third leader, takes over from Godane, who had succeeded top commander Ismail Arale. Arale was sent to Guantanamo Bay in 2007.

The armed group also confirmed the death of two other unnamed officials.

The US confirmed on Friday that Godane was killed in September 1 airstrike which happened outside the port city of Barawe - al-Shabab's last major stronghold.

The attack on Tuesday targeted vehicles and encampment near Barawe, the armed group's base of operations. The US had said the targets were senior commanders of al-Shabab.

In 2012 the US put a reward of $7m for information leading to the capture or killing of Godane.

Also on Friday, president Hassan Mohamud issued a statement calling for fighters to leave the rebel group. He offered a 45-day amnesty to fighters who renounced the group.

Godane, also known as Abu Zubeyr, rose to the top of the al-Qaeda-linked group in 2008 following the death of his predecessor Aden Hashi Ayro in a US airstrike in the southern town of Dhusamareeb.

Under his stewardship the rebel group pledged allegiance to al-Qaeda in 2009 and started focusing their attacks outside Somalia.

Deadly attacks

Last year, the group's fighters stormed a shopping mall in the Kenyan capital, Nairobi, killing at least 67 people.

In 2010, al-Shabab carried out an explosion in Kampala that left 78 football fans dead during the world cup. The attack was the first by the group outside Somalia.

On Saturday, Somalia's government warned of a wave of retaliatory attacks by the group following Godane’s death.

"Security agencies have obtained information indicating that al-Shabab is now planning to carry out desperate attacks against medical facilities, education centres and other government facilities," Somalia's national security minister, Kalif Ahmed Ereg, said in the capital Mogadishu.

"The security forces are ready to counter their attacks and we call on people to help the security forces in standing against violent acts," he added.

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