The State Department warned U.S. citizens Wednesday to consider carefully plans to travel to Pakistan.
Unconfirmed reports that Americans have been targeted for kidnapping or other terrorist actions were cited by the department in a statement to the media and to U.S. diplomats.
The warning cited the disappearance of Wall Street Journal reporter Daniel Pearl.
An unsigned e-mail purportedly sent by his kidnappers threatened to kill Pearl within 24 hours and warned American journalists to leave Pakistan within three days or "be targeted."
The unsigned e-mail claimed that Pearl, who disappeared in Karachi on Jan. 23, was an agent of the Israeli intelligence service, the Mossad.
On Monday, all U.S. Embassy and consulate workers and their families were authorized to return to their posts.
But the warning issued Wednesday said "ongoing events in the region require that all Americans in or traveling through Pakistan closely consider the implications for their security and safety."
There is support for the Taliban militia and the Al Qaeda network among the Pakistani people, the statement said.
Americans who live in Pakistan or visit the country "should exercise maximum caution and take prudent measures," the department said.