Obama says US may take further military action to protect Americans in South Sudan In this photo taken Wednesday, Dec. 18, 2013 and released by the U.S. Air Force, U.S. Army soldiers of the East Africa Response Force (EARF), a Djibouti-based joint team assigned to Combined Joint Task Force-Horn of Africa, prepare to load onto a U.S. Air Force C-130 Hercules at Camp Lemonnier, Djibouti, to support with an ordered departure of personnel from Juba, South Sudan.AP/U.S. Air Force President Barack Obama told Congress Sunday that he may take further military action to protect Americans trying to evacuate violence-plagued South Sudan. In a letter to Congress, Obama said that about 46 U.S. troops were deployed Saturday to help evacuate Americans. That's in addition to another 45 troops deployed to reinforce the U.S. Embassy in Juba. Four U.S. troops were injured in the evacuation mission Saturday when gunfire hit three military planes in Bor. All four are in stable condition, the White House said. It remains unclear how many Americans are still stranded in Bor and other rural towns. Obama is on his annual vacation in Hawaii, but he said in the letter to congressional leaders that he's monitoring the situation. Earlier Sunday, Obama was briefed by advisers on events in South Sudan following a meeting that his national security adviser, Susan Rice, held with national security aides and U.S. personnel still in South Sudan. "I may take further action to support the security of U.S. citizens, personnel, and property, including our Embassy, in South Sudan," Obama wrote. The U.S. and other countries have been evacuating their citizens from South Sudan. The U.S. has evacuated about 680 Americans and other foreign nationals so far, State Department spokeswoman Jen Psaki said in a statement. Fighting continued Sunday in South Sudan, where the country's central government lost control of the capital of a key oil-producing state, the military said, as renegade forces loyal to a former deputy president seized more territory. The unrest has raised fears of full-blown civil war. Bentiu, the capital of oil-rich Unity state, is now controlled by a military commander loyal to former Vice President Riek Machar, said Col. Philip Aguer, the South Sudanese military spokesman. "Bentiu is in the hands of a commander who has declared support for Machar," he said. Bentiu is not in our hands. The armed rebels were said to be in control days earlier of some of South Sudan's oil fields, which have historically been a target for rebel movements, endangering the country's economic lifeblood. South Sudan gets nearly 99 percent of its government budget from oil revenues, and the country reportedly earned $1.3 billion in oil sales in just five months this year, according to the London-based watchdog group Global Witness. Although the country's capital, Juba, is mostly peaceful a week after a dispute among members of the presidential guard triggered violent clashes between military factions, fighting continues as the central government tries to assert authority in the states of Unity and Jonglei. Bor, the capital of Jonglei, is said to be the scene of some of the fiercest clashes between government troops and rebels. Michael Makuei Lueth, South Sudan's information minister, said Machar was believed to be hiding somewhere in Unity state. He is a rebel, he's a renegade and we are looking for him. He's moving in the bushes of South Sudan," Lueth said of Machar. The U.N. Mission in South Sudan said in a statement Sunday that all non-critical staff members in Juba are being evacuated to Uganda. The mission said the move was "a precautionary measure to reduce pressures on its limited resources" as it continues to provide assistance and shelter to more than 20,000 civilians gathered inside its compounds in Juba, the mission said in a statement. Hilde Johnson, the U.N. secretary-general's envoy in South Sudan, said the evacuation doesn't mean the U.N. is "abandoning" South Sudan. "We are here to stay, and will carry on in our collective resolve to work with and for the people of South Sudan," she said. To anyone who wants to threaten us, attack us or put obstacles in our way, our message remains loud and clear: we will not be intimidated. Hundreds have been killed in the fighting and U.N. Secretary-General Ban Ki-moon on Sunday urged South Sudan's leaders "to do everything in their power" to stop the violence. Foreign ministers from neighboring countries Kenya, Ethiopia, Uganda and Djibouti were in South Sudan earlier in the week to try and diffuse the crisis. South Sudan, which became independent in 2011 after decades of a brutal war with Sudan, has been plagued by ethnic discord, corruption and conflict with Sudan over oil revenues. Although the south inherited three-quarters of Sudan's oil production when it declared independence in 2012, its oil exports are pumped through pipelines running north, raising concern a rebel takeover of southern oil fields could invite Sudan into the conflict.