Gunmen wearing police uniforms stopped a passenger bus in the southern Philippines early on Thursday and fatally shot four people, including two police marshals, officials said.
The attack came just three days after a disgruntled ex-policeman took hostage a busload of Hong Kong tourists in Manila, killing eight of them before he was shot dead by a police sniper.
Police said the bus attacked on Thursday was travelling from Cagayan de Oro to Zamboanga city in the southern Philippines where Muslim rebels and criminal gangs operate when it was flagged down at a road checkpoint by gunmen in police uniforms.
They ordered the passengers to get off and then fatally shot four people, including two uniformed police marshals, said Orlando Vinas, the Lanao del Norte provincial police chief.
Vinas said the marshals were deployed on regional buses plying the same highway after gunmen early this month held up a bus to extort money from passengers.
Highway robberies are common in the country's south, where the proliferation of illegal firearms and weak law enforcement have fueled the Muslim separatist insurgency and instability for decades.