NAIROBI, Kenya — A Kenyan official says suspected al-Shabab gunmen killed at least six people when they shot at two buses traveling in Mandera County in northern Kenya.

North Eastern Regional Security Coordinator Mohammed Saleh said Friday several people were wounded in the attack as the buses headed toward Mandera town at 10 a.m. The buses were carrying passengers from the capital, Nairobi.

Last week five police officers were killed in the same area in an attack blamed on the Somalia-based al-Shabab extremist group. Al-Shabab has vowed retribution on Kenya for sending troops in Somalia since 2011 to fight the militants who are waging an insurgency against Somalia's weak western-backed government.

The Kenyan government said it had deployed its troops because of cross-border attacks by al-Shabab and the kidnapping of four Europeans.