The US embassy in Manila released a statement calling for US citizens to exercise extreme caution and consider cancelling American gatherings in the sprawling and chaotic megacity of 15 million people.

"The US Embassy advises American citizens in the Philippines that a threat against American citizens in metropolitan Manila, specifically the Pasay City neighborhood, has been detected by reliable security forces," the statement said.

"This threat remains in effect until October 10, 2012," it said.

The statement said US Ambassador Harry Thomas had ordered stepped-up security for all US government facilities in Manila.