MANILA - The Metro Rail Transit and the Light Rail Transit announced Thursday that bottled drinks are prohibited inside train stations as part of tighter security measures following bombing incidents in Jolo, Sulu.‬

In a statement, the transportation department said the measure seeks to prevent entry of dangerous chemicals contained in plastic bottles as these could be used as "liquid bombs."

"Hence, we encourage our passengers before entering MRT-3 stations to consume their bottled drinks, otherwise they will not be allowed to set foot in," it said.

The agency urged commuters to cooperate and remain vigilant.

"Rest assured that we are doing our best to ensure safety and security of our passengers, as these two are our topmost priorities," it said.

At least 21 people were killed while several others were injured after twin blasts went off during a Sunday mass in the Our Lady of Mount Carmel Cathedral in Jolo, Sulu.

• Several killed, dozens wounded in twin blasts at Jolo church

--Report from Jacque Manabat, ABS-CBN News