The United States on Tuesday offered a US$5 million reward for information on the whereabouts of a leader of the Somali jihadist group Al-Shabaab.

Abukar Ali Adan, who was officially designated a terrorist by the United States in 2018, is the deputy commander of the extremist group, the State Department said.

It said he is also associated with several branches of Al-Qaeda.

Al-Shabaab has been waging a violent insurgency against the government of Somalia for more than 15 years.

Flushed out of major cities in 2011-2012, the organization is still present over vast rural areas, mainly in the center and south of the Horn of Africa nation. It stages attacks regularly against security forces, politicians and civilians.

Somali President Hassan Sheikh Mohamud took office in May last year vowing all-out war against the militants, who have also carried out deadly attacks in neighboring countries including Kenya.

"The terrorist group continues to plot, plan, and conspire to commit terrorist acts against the United States, US interests, and foreign partners," the State Department said.

Meanwhile in Kenya, the interior minister on Tuesday vowed an "all-out war" against the militants and named 35 people suspected to be members of the group and involved in attacks near the country's porous border with Somalia.

"The 35 terrorists being sought are connected with the placement of Improvised Explosive Devices (IEDs) on various earth roads in the area," Kithure Kindiki said in a statement.

"While the arrest or neutralization of these terrorists remains the most immediate objective, other measures and strategies have been put in place to win the war against terror," Kindiki said.

Kenya is a major contributor to the African Union force backing Somalia's central government in Mogadishu in its fight against Al-Shabaab and has suffered a string of deadly retaliatory assaults.

The country last month marked the 10th anniversary of a siege at the upmarket Westgate shopping centre in the capital Nairobi in 2013 that killed 67 people.

Two years after Westgate, Al-Shabaab fighters attacked Garissa University in eastern Kenya, killing 148 people, almost all students.

It was the second most deadly attack in Kenya's history, surpassed only by Al-Qaeda's bombing of the US embassy in Nairobi in 1998 that killed 213 people.

In 2019, Al-Shabaab gunmen killed 21 people at the upscale Dusit hotel complex in Nairobi. (AFP)