Colombo, May 13 (IANS): Sri Lanka on Monday declared a nationwide curfew after anti-Muslim riots spread to several parts of the island nation, just three weeks after multiple suicide bombings claimed by the Islamic State claimed over 250 people.

The curfew will remain in force till 4 a.m. Tuesday, said police spokesman S.P. Ruwan Gunasekara.

Prime Minister Ranil Wickremesinghe in a statement urged citizens to remain calm and not be swayed by rumours, Colombo Gazette reported.

"Security forces are working tirelessly to apprehend terrorists and ensure the security of the country but each time there is civil unrest we increase their burden and hamper ongoing investigations," he said.

Authorities had earlier in the day reimposed curfew in several towns of the still shaken Buddhist-majority country and temporarily blocked social media websites after a Facebook post sparked the anti-Muslim riots.

Sri Lanka has been on the edge since the April 21 horror when suicide bombers exploded themselves in three churches and three luxury hotels besides two other places. Most targets were in Colombo.

The killings were the worst to hit Sri Lanka since the end of the civil war a decade earlier.

On Sunday, a group of people stormed into Chilaw town following the Facebook post by a Muslim shopkeeper about "an attack plan".

Reports said that several people threw stones at mosques and attacked Muslim-owned shops. A man, presumably Muslim, was beaten up.

The same day, a curfew was imposed in Chilaw and Kuliyapitiya, Bingiriya, Hettipola and Dummalasuriya areas and the social media ban was imposed to prevent circulation of fake news and incitement to violence.

The curfew was then extended to Rasnayakapura and Kobeigane areas and authorities warned that "tough action will be taken against anyone disrupting the peace".

The suicide bombings and lingering tensions have affected tourism, a major foreign exchange earner for Sri Lanka.