MANILA - The Bureau of Immigration said Tuesday it was on heightened alert against the possible entry of foreign terrorists after a powerful explosion ripped through a Catholic church in the south, killing 21 people and wounding some 100 others.

Immigration officers at international airports and seaports were ordered to be "doubly vigilant in screening arriving foreign travelers and to make sure that they have legitimate purposes in entering the country," the agency said in a statement.

"Those who cannot satisfactorily explain their purpose in coming here are to be turned back and booked on the first available flight to their port of origin," said BI Commissioner Jaime Morente.

The travel control and enforcement unit, which does the secondary inspection of passengers with doubtful documents and travel purpose, and the border control and intelligence unit, which monitors passengers acting suspiciously, were also instructed to be extra vigilant, said BI Port Operations Division Chief Grifton Medina.

Local and international intelligence agencies share their database with BI to help detect suspected terrorists, Medina said.

At least 21 people perished and about 100 others were injured when twin explosions rocked the Our Lady of Mount Carmel Cathedral during Sunday mass.

The Armed Forces of the Philippines is convinced that an ISIS-inspired Abu Sayyaf Group is responsible for the twin bombings. The international terror group earlier claimed responsibility for the incident.