The travel warning also said the department has authorized family members of U.S. Embassy staff and non-essential personnel to leave the country.
The advisory cited terrorism and civil unrest in the impoverished Arab nation and said that in the event of a crisis evacuation options would be severely limited due to "the lack of infrastructure, geographic constraints, and other security concerns."
The State Department said the threat level in Yemen, where demonstrators are calling on the country's besieged president to leave office, is "extremely high."
"Piracy in the Red Sea, Gulf of Aden and Indian Ocean is also a security threat to maritime activities in the region. Terrorist organizations continue to be active in Yemen, including Al-Qaeda in the Arabian Peninsula," the travel warning said.
The State Department said the U.S. government remained concerned about possible attacks against American citizens, facilities, businesses, and perceived U.S. and Western interests.
Suspected al-Qaida fighters ambushed and killed four Yemeni soldiers on Sunday who were distributing food to other troops manning checkpoints in a remote, mountainous region of the country.
Travel Advisory: http://tinyurl.com/ct3c9d