At least four people have been killed and several injured by a grenade attack on a bus in the Kenyan capital, Nairobi, the interior ministry says.
The blast hit the 32-seater bus near the Eastleigh suburb, which is home thousands of ethnic Somalis.
No group has said it was responsible for the attack.
But the Eastleigh area has suffered several attacks in the past blamed by officials on Somalia's al-Qaeda-linked al-Shabab militant group.
The group was behind the four-day siege at the Westgate shopping centre in Nairobi in September in which 67 people died.
"A group of assailants threw a grenade into a bus with 32 seats," the interior ministry said on its Twitter account.
"The attack is an unfortunate and cowardice incident which will not be tolerated," it said, appealing for information from the public.
Police said the bus had been close to a girls' school when it was hit on Saturday afternoon.
The Kenyan Red Cross said on its Twitter feed that 15 injured people had been taken to hospital.
Last year an attack on a minibus in Eastleigh - often referred to as Little Mogadishu for its high number of Somali residents - killed at least seven people.
Kenya sent its troops to fight al-Shabab in Somalia last year. They have now joined the 18,000-strong African Union (AU) force supporting the UN-backed government.
Kenya has been struggling to deal with the vast numbers of Somalis who have fled the poverty and violence in their home country since the 1990s and are living in and around the capital.
Many are living in huge makeshift camps, including around half a million in Dadaab, the world's largest refugee camp.
Earlier this year, Kenya announced planed to begin voluntarily repatriating some of the refugees.
Correspondents say that among humanitarian concerns, Kenya fears the camps could be sheltering militants plotting attacks for al-Shabab and other groups.