Several Christian families have fled their homes in northwestern Pakistan after their minority community received threatening letters to shut churches and convert to Islam, a rights activist said Friday.

The unsigned letters were distributed this week in the town of Charsadda, where Pakistan’s interior minister last month escaped a suicide attack that killed 28 people.

Shahbaz Bhatti, head of the All Pakistan Minorities Alliance, claimed militants trying to impose Taliban-style social strictures were responsible for the letters, which gave Christians until May 17 to convert.

Police said Thursday they had stepped up security at churches in Charsadda. Copies of the handwritten letter, sent to churches and homes, did not say what consequences the Christians might face if they did not comply with the ultimatum.

The threats come amid a spate of reports about how religious vigilantes are using intimidation and violence to enforce a strict interpretation of Islam across an expanding swath of Pakistan near the border with Afghanistan.

Christians and other minorities make up about 3 percent of Pakistan’s overwhelmingly Muslim population of some 160 million.