The State Department issued new guidance Friday urging U.S. citizens to leave Sri Lanka in the wake of a terrorist attack on Easter on churches and luxury hotels that killed more than 250 people.

The message, which urged the voluntary departure of all U.S. citizens from the country, also ordered that all school-age family members of U.S. government employees leave Sri Lanka.

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"Terrorist groups continue plotting possible attacks in Sri Lanka. Terrorists may attack with little or no warning, targeting tourist locations, transportation hubs, markets, shopping malls, government facilities, hotels, clubs, restaurants, places of worship, parks, major sporting and cultural events, educational institutions, airports, hospitals, and other public areas," the State Department said on its website.

The guidance came the same day a Sri Lankan raid on a suspected militant hideout killed at least 15 people.

The raid on the country’s eastern coast followed a shootout nearby, after which authorities found explosives, detonators, “suicide kits,” military uniforms and ISIS flags.

ISIS has claimed responsibility for the attack, though local extremist group National Thowheed Jamath (NTJ) has been blamed for Sunday's bombings.

Catholic leaders on Friday morning also announced that Sunday Masses across Sri Lanka would be canceled indefinitely as the country grapples with the ongoing security threat.

"Everyone is nervous," one Colombo resident told The Associated Press. "Not just the Muslims. Buddhists, Christians, Hindus — everybody’s nervous."