(CNN) -

Manila's international airport is on high alert after authorities foiled an alleged bomb plot Monday, state-run media in the Philippines report.

Counter-terror agents acting on a tip interrupted a group of men allegedly assembling improvised explosive devices in a white truck parked outside a terminal at Ninoy Aquino International Airport, said the Philippines News Agency, or PNA.

The agency initially said four suspects had been arrested but later revised the number of arrests to three. The white truck had been parked outside terminal three of the airport when it was intercepted about 1:45 a.m. local time (1:45 p.m. Sunday ET), PNA said.

The agents from the Philippines National Bureau of Investigation also seized firearms from the men, it said.

The names of the suspects have not yet been released, and investigators are trying to establish who was behind the alleged plot and their motive. The investigation is continuing.

Rebel groups in the Philippines have been fighting to establish an independent Muslim homeland in the south of the predominantly Christian country.