KAMPALA, Uganda - South Sudan's central government lost control of the capital of a key oil-producing state Sunday, the military said, as renegade forces loyal to a former deputy president seized more territory in fighting that has raised fears of full-scale civil war in the world's newest country.

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 revenue, 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.

The U.N. Mission in South Sudan said in a statement Sunday that all noncritical staff members in Juba were being evacuated to Uganda.

The United States and other countries have been evacuating their citizens from South Sudan. The United States has evacuated about 680 Americans and other foreign nationals, State Department spokeswoman Jen Psaki said.

President Obama told U.S. Congress he may take further military action to protect Americans in South Sudan. In a letter to Congress, he said that about 46 U.S. troops were deployed Saturday to help evacuate Americans. That is in addition to 45 troops deployed to reinforce the U.S. Embassy in Juba.

"I may take further action to support the security of U.S. citizens, personnel, and property, including our Embassy, in South Sudan," Obama wrote.

On Saturday, gunfire hit three U.S. military aircraft trying to evacuate American citizens in Bor, wounding four U.S. service members. The troops are in stable condition, the White House said.

It remains unclear how many Americans are stranded in Bor and other rural towns.