The State Department has ordered an immediate evacuation of U.S. citizens and government personnel out of Yemen because of security risk and the possibility of a terrorist attack.

“… We are concerned about a threat stream indicating the potential for terrorist attacks against U.S. persons or facilities overseas, especially emanating from the Arabian Peninsula,” State Department spokesman Jen Psaki said. “As such, the department is taking appropriate steps to protect our employees, including local employees and visitors to our facilities.”

The security situation in Yemen currently is considered “extremely high risk” — especially for U.S. citizens, prompting the State Department to evacuate all but the most critical embassy staff in the country. In a travel warning issued Tuesday morning, State also urged U.S. nationals who are living in the country to “depart immediately.”

Pentagon spokesman George Little told CBS News that the U.S. Air Force had already started airlifting some emergency personnel from Sana’a Tuesday morning. He also said the Department of Defense will continue to have personnel on the ground in Yemen to support the State Department and “monitor the security situation.”

The country-wide evacuation, which has no end time, is rare. U.S. institutions in foreign countries flee when there is no alternative to secure the safety of U.S. citizens. For any citizens and staff remaining in Yemen, the State Department advises them to limit their movement.

In addition to curtailing travel within the country, Psaki said U.S. citizens remaining in Yemen should be aware of their surroundings “whether in residences or moving about,” make their own emergency plans, enroll their presence in the country through a State Department-sponsored traveler enrollment program and provide current contact information and “next-of-kin” or emergency contact information.

The U.S. government also advises U.S. citizens not to travel to Yemen or any “high-risk” countries through the end of August.

Previous reports about the threat, including one in the New York Times, say the evacuation is the latest move to protect U.S. citizens abroad after intelligence officials intercepted a broad order delivered by Al Qaeda leader Ayman al Zawahiri to carry out attacks in Yemen.

Over the weekend, the State Department closed 22 diplomatic facilities across the Middle East, Northern Africa and the Arabian Peninsula after intercepting what U.S. government officials say is the highest level of terrorist “chatter” since 9/11. They reopened a handful of those facilities Monday and ordered that the rest remain closed until at least Saturday.

U.S. authorities overheard al Zawahiri, who is believed to be hiding along the Pakistan-Afghanistan border, and Nasir al-Wuhayshi, the leader of the Yemen-based al Qaeda in the Arabian Peninsula (AQAP), discussing something “big,” CBS News reported.