The Marine Corps has sent a team of security guards from the Corps' embassy reinforcement team to the U.S. Embassy in Juba, South Sudan, a U.S. official said.

The Marine Security Augmentation Unit was created after the September 2012 attacks in Benghazi, Libya, that killed four Americans, including Ambassador Christopher Stevens.

The U.S. official would not say exactly how many Marines have been sent to Juba, where fighting between rival factions has killed hundreds of people recently.

"The size of the MSAU is determined based on embassy protection requirements determined by the Department of State; we will not discuss specific number," the U.S. official said,

On Tuesday, a spokeswoman for U.S. Africa Command said that a crisis response force of nearly 40 troops arrived at the embassy,"to temporarily assist the U.S. Embassy in bolstering its security and assisting with a reduction of non-emergency personnel," adding the troops had brought several vehicles to secure the embassy.

The State Department has chartered two flights for U.S. citizens who want to leave South Sudan, the embassy announced Wednesday on its Facebook page. The planes will leave for Entebbe, Uganda, on Thursday.

Based in Quantico, Virginia, the entire MSAU has about 120 Marine security guards. The unit typically dispatches in squad-sized teams, but the deployment to Juba would represent about one third of the entire force. 

Unlike other Marine Corps units that can quickly reinforce security at U.S. diplomatic posts, the MSAU consists entirely of trained embassy guards. That means it can be summoned by a U.S. ambassador, chief of mission or regional security officer if a diplomatic post anywhere in the world is under threat. The MSAU can also be dispatched to reinforce Marine security guards already at an embassy if there is credible intelligence of an impending attack.

Following November's terrorist attacks in France that killed more than 129 people, the Marines sent a MSAU team to the U.S. embassy in Paris. At the time, the unit had deployed on about 60 missions across the world since being established little more than two years earlier