From the U.S. State Department:
The Department of State warns U.S. citizens to defer all non-essential travel to Pakistan. ...
Up-to-date information on security can also be obtained by calling 1-888-407-4747 toll-free in the United States and Canada or, for callers outside the United States and Canada, on a regular toll line at 1-202-501-4444. These numbers are available from 8:00 a.m. to 8:00 p.m. Eastern Time, Monday through Friday (except U.S. federal holidays).
For further information, U.S. citizens traveling abroad should consult the Department of State's Country Specific Information for Pakistan. Stay up to date by bookmarking our Bureau of Consular Affairs website, which contains the current Travel Warnings and Travel Alerts as well as the Worldwide Caution.
U.S. State Department Warning