From the U.S. State Department:

The Department of State warns U.S. citizens of the high security threat level in Yemen due to terrorist activities and civil unrest. The Department urges U.S. citizens not to travel to Yemen. U.S. citizens currently in Yemen should depart. Effective September 1, 2011, the Department of State lifted the Ordered Departure status for U.S. government employees at the U.S. Embassy in Sana'a. The embassy remains a restricted staffing post. As staff levels at the embassy are restricted, our ability to assist U.S. citizens in an emergency remains limited and may be further constrained by the fluid security situation. ...

U.S. State Department Warning