Christians in northwest Pakistan have received threatening letters warning them to shut their churches and convert to Islam, officials said Thursday, the latest sign of how religious extremists are trying to police society.

Copies of the handwritten letter were delivered to two churches and several Christians' homes in Charsadda, a northwestern Pakistan town where the federal interior minister last month escaped a homicide attack that killed 28 people.

Christians have alerted police to the letters and security has been stepped up at churches, said a local police official, Ali Haider.

Police are investigating who sent the unsigned notes, which gave the Christians 10 days to convert, Haider said.

He said the letter did not say what consequences they might face if they did not comply with the ultimatum, which expires May 17.

The threats come amid a spate of reports about how religious extremists are trying to impose Taliban-style social strictures across an expanding swath of Pakistan near the border with Afghanistan.

Other recent examples are bombings of music stores — including two blasts in Charsadda last week — threats to barbers not to shave customers' beards and pressure for the closure of schools for girls.

Iqbal Khan, another local police official, said a small bomb tied to a motorcycle exploded in Charsadda late Wednesday, damaging several CD shops. He said authorities had yet to make any arrests over the bombings.

Asif Daudzai, minister for information in North West Frontier Province, which includes Charsadda, said authorities would uphold minorities' freedom of religion.

"No one will be allowed to do it (force them to convert). Christians and other minorities are free to go to churches and temples and live according to their religion," Daudzai said.

Christians and other minorities make up about 3 percent of Pakistan's overwhelmingly Muslim population of some 160 million, and about 500 Christians coexist peacefully with Muslims in Charsadda.

Christians have been targeted in several attacks since Pakistan became a key ally of Washington in its campaign against terror following the Sept. 11, 2001, suicide hijackings on the U.S., and the invasion of Afghanistan.

In 2002, five people were killed, two of them Americans, when suspected Islamic militants set off grenades at a church in Islamabad's heavily guarded diplomatic enclave.