Kenya's Red Cross said gunmen opened fire on a bus Friday, killing at least seven people.
The aid group says the attack happened near Witu, a town in Lamu county.
Al-Qaida-linked al-Shabab militants have claimed responsibility for the assault.
The militants said the attack was carried out in response to the government's assertion that all the areas that have recently been subjected to attacks are now secured, following the deployment of government troops.
The insurgents have vowed to carry out attacks on Kenya to avenge the presence of Kenyan troops fighting the militants in Somalia where Kenyan soldiers are part of an African Union peacekeeping force.