At least six people have been killed when Islamist militants opened fire on buses in northern Kenya.

The buses were travelling to the town of Mandera, near the border with Somalia, when they were attacked.

Somalia-based terrorist group al-Shabaab has claimed responsibility.

At least two people were injured, and a police officer is among the fatalities.

Mandera has been the scene of frequent al-Shabaab attacks, which have killed dozens of civilians and security personnel.

Buses were a target in a November 2014 attack, in which 28 people were killed when the Islamists singled out Christian passengers.

On Thursday, the US State Department issued a warning saying that attacks by al-Shabaab were imminent.

The warning was labelled unfair and dismissed by Kenyan authorities only hours before the latest attack happened.