In a letter to House and Senate leaders, the president said he was keeping congressional leaders informed on U.S. military activities in South Sudan, "consistent with the War Powers Resolution."
Obama notified Congress Thursday he had sent U.S. forces "to support the security of U.S. personnel and our Embassy in South Sudan."
The letter Sunday included formal notification of an attack on U.S. aircraft engaged in evacuating U.S. citizens in the town of Bor.
"After the aircraft came under fire as they approached Bor, the operation was curtailed due to security considerations, and the aircraft and all military personnel onboard departed South Sudan without completing the evacuation," he said.
"The purpose of this operation was to protect U.S. citizens, personnel, and property," the president said. "As I monitor the situation 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 said the action was "consistent with my responsibility to protect U.S. citizens both at home and abroad, and in furtherance of U.S. national security and foreign policy interests, pursuant to my constitutional authority to conduct U.S. foreign relations and as Commander in Chief and Chief Executive.
"I appreciate the support of the Congress in these actions."
A senior U.S. official told CNN U.S. military aircraft would be back in service in South Sudan before the end of the weekend.
Four U.S. service members were wounded Saturday when a trio of CV-22 Ospreys took ground fire during a relief flight into the town of Bor, but the situation in the area had stabilized enough Sunday that U.N. helicopters were operating normally.
"If U.N. helicopters are flying, then there's no reason the U.S. military can't go back in," the official said.
Officers at U.S. Africa Command were taking the risk of future rebel fire into consideration when planning the evacuation flights.
A top priority for U.S. planners will be the evacuation of Americans in the Bor area, which is a hot point for violence between South Sudan's fledgling government and rebel troops the government says are loyal to the country's former vice president.
Fighting during the past week cost hundreds of lives and sent as many as 40,000 refugees to the relative safety of U.N. camps, CNN said.
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