Philippine police have rescued 80 Filipino women from a suspected human trafficking syndicate that was planning to send them to be maids in Malaysia and the Middle East, including Lebanon.

Police Superintendent Celso Bael said they stopped the women, who pretended to be tourists, from boarding a chartered flight from the southern city of Zamboanga to Malaysia on Thursday.

At least four people were killed and 15 injured when a bomb exploded on a crowded bus in the Philippine capital's financial district on Tuesday, authorities said.

The attack occurred two months after the United States and several other Western governments warned a terrorist attack in Manila was imminent, and followed a bus hijacking last year that left eight Hong Kong tourists dead.