The US has imposed sanctions on two senior leaders of Islamist militant group al-Shabab accused of responsibility in an attack on a military base in Kenya.

One US military service member and two contractors were killed in the January attack in Camp Simba on Manda Island. It was the first attack by al-Shabab on US forces in Kenya.

The State Department said it had designated Abdullahi Osman Mohamed and Maalim Ayman as Specially Designated Global Terrorists.

It said Mr Mohamed was al-Shabab’s senior explosives expert and leader of its media wing, while Mr Ayman was responsible for planning the attack on Camp Simba.

"US persons are generally prohibited from engaging in any transactions with Mohamed and Ayman. Their property and interests in property subject to US jurisdiction are blocked," the state department said in a statement.