Khartoum and London, Asharq Al-Awsat—Three US military aircraft attempting to evacuate US citizens from South Sudan came under fire on Saturday, wounding four US soldiers as the conflict in South Sudan continued to escalate.

Three CV-22 Ospreys were fired on by unknown attackers as they approached the city of Bor, which has been occupied by forces loyal to former Vice President Riek Machar, forcing the mission to be aborted. An unknown number of US citizens remain trapped in the world’s newest country after forces loyal to Machar have sought to take control of South Sudan.

A South Sudanese military official confirmed to Asharq Al-Awsat that a US military plane was transferring US soldiers to the city of Bor, the capital of Jonglei state, when it was fired upon by unknown attackers. The official said it was unclear whether the shots were fired by supporters of Riek Machar or soldiers loyal to the Juba government.

Speaking to the BBC on Saturday, Machar confirmed that much of the country is under rebel control, including the city of Bor, the strategic town where the US aircraft had attempted to land on Saturday.

Earlier Machar had called on the government and military to remove President Salva Kiir from his position, accusing him of seeking to incite an ethnic conflict in the country.

South Sudanese military spokesman Colonel Philip Aguer confirmed that government troops are no longer in control of the capital of Jonglei state, blaming the attack on the US aircraft on “renegade” soldiers.

“Bor is under the control of the forces of Riek Machar,” he said.

Elsewhere, eyewitnesses in the South Sudanese town of Bentiu, located in Unity State, informed Asharq Al-Awsat that soldiers loyal to Machar had completely taken over the town.

Speaking over state radio on Saturday morning, Fourth Division Commander of the South Sudan Army, James Koang Chuol, announced that he had overthrown Caretaker Governor Joseph Nguen Monytuel and that his forces are no longer loyal to President Salva Kiir. He also claimed that the South Sudanese military had tried to assassinate him on orders from senior members of the military in Juba.

However in exclusive comments to Asharq Al-Awsat, Colonel Aguer denied that Unity State was completely in rebel hands. Aguer acknowledged that James Koang Chuol has defected, but denied that all those under his command had also joined Machar’s forces. He said that government forces remain in control of a number of key oil production sites in Unity State.

The latest violence comes following tensions among senior members of South Sudan’s ruling party, the Sudanese People’s Liberation Movement (SPLM). South Sudan President Salva Kiir accused opponents within the SPLM of seeking to stage a coup against him, ordering the arrest of 11 senior figures, including Machar, who is currently on the run, with his supporters reportedly taking control of large portions of South Sudanese territory.

Reports indicate that the conflict has ethnic features, with many world leaders warning against South Sudan falling prey to full-scale ethnic civil war. President Kiir is a member of the majority Dinka ethnic group, while Machar is being supported by members of his own Nuer community.

US President Barack Obama has warned that any move to take power by military means would lead to an end of US and international support for South Sudan, while UN Secretary-General Ban Ki-Moon has called on South Sudan’s leaders “to do everything in their power to ensure that their followers hear the message loud and clear that continued violence, ethnic and otherwise, is completely unacceptable and pose a dangerous threat to the future of their country.”