HONOLULU - President Barack Obama told Congress Sunday that he may take further military action to protect Americans in 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. All four are in stable condition, the White House said.

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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.

Fighting continued Sunday in South Sudan, where the central government has lost control of the capital of a key oil-producing state. The unrest has raised fears of full-blown civil war.