The U.S. State Department issued a travel warning for Yemen on Friday, citing a "high security threat level in Yemen due to terrorist activities and civil unrest." The warning urges U.S. citizens to defer any travel to Yemen and asks Americans currently living in the country "to depart when you are safely able to do so." The warning comes at a time when the country is further descending into chaos amid a civil war and is facing Saudi Arabia-led airstrikes aimed at Houthi rebels who seized control of the government in February. 

The U.S. government moved its embassy staff out of Yemen after suspending embassy operations in the capital, Sanaa, in February. It also airlifted military personnel in March. But the State Department still has no plans to lead a government-coordinated evacuation of U.S. citizens living in Yemen at this time.

"We encourage all U.S. citizens to shelter in a secure location until they are able to depart safely," the State Department said in its travel warning on Friday. "U.S. citizens wishing to depart should do so via commercial transportation options when they become available." 

The first casualty of a U.S. citizen in the conflict occurred on Tuesday, when Yemeni-American Jamal al-Labani of Oakland, Calif., was hit by mortar shells, reports Al Jazeera. Americans trapped in Yemen have called on the U.S. State Department to help them evacuate, as they seek escape routes out of the country. 

Meanwhile, other countries already have evacuated their citizens and others. China helped some 225 non-Chinese citizens evacuate the country, including British, German and Canadian citizens. India evacuated 351 of its citizens on Friday. 

The State Department said Americans may seek help from other governments in getting out of Yemen, but that there are no guarantees. "Additionally, some foreign governments may arrange transportation for their nationals and may be willing to offer assistance to others," the State Department warning read.

"There is no guarantee that foreign governments will assist U.S. citizens in leaving Yemen," the warning continued. "U.S. citizens who choose to seek foreign government assistance in leaving Yemen should only do so if they can safely make their way to the point of embarkation and have received confirmation that there is space available. Even if assured there is space aboard transportation, U.S. citizens should be aware that there is no guarantee that they will be permitted to board the transport, or may have to wait an indefinite period until they can do so."