The rising civil unrest in South Sudan has cost the U.S. its first casualties. Rebel fire badly damaged two American aircraft carrying troops, injuring four, one critically.

Initial reports by the Associated Press said the aircrafts were flying to Bor, the rebel-conquered capital of what is one of South Sudan’s most fraught states right now, when they were attacked by alleged rebel fire. The military later confirmed to Reuters early Saturday morning that the mission was intended to evacuate American citizens in the country.

U.S. Africa Command released this short statement about the attack:

"After receiving fire from the ground while approaching the site, the aircraft diverted to an airfield outside the country and aborted the mission.”

The injured are currently receiving medical attention in Kenya, according to the Associated Press.

This takes things from bad to worse in a country that experts say may be headed toward ethnic cleansing. The tensions in the world’s youngest country (it split from Sudan in 2011) escalated significantly with last weekend’s alleged coup attempt on President Salva Kiir by his former vice-president, Riek Machar. Kiir belongs to the majority Dinka; Machar belongs to the second-largest majority, the Nuer, which makes up the predominance of the rebel groups. In an interview with Al-Jazeera, Machar denies the coup attempt, saying Kiir is using him as “a scapegoat” to purge political rivals. Human Rights Watch reported that Dinka soldiers targeted Nuer civilians in the capital, Juba.

Negotiations are ongoing, though Machar appears set in his demands that Kiir leave power. 

International forces haven’t proven immune to attack, either. On Thursday, three United Nations peacekeepers from India were killed in a base housing civilians, prompting the UN to warn that the country may be on the brink of civil war. The BBC reports that more than 500 have died in the fighting. 

Americans, too, have ratcheted up their support for the country over the last week. The U.S. has already sent 45 service members and a special envoy to the country. On Thursday, President Barack Obama spoke out against the fighting.