HONOLULU — President Barack Obama told congressional leaders Sunday that he was closely monitoring the unrest in South Sudan, following an attack that wounded four U.S. service members, and said he “may take further action to support the security of U.S. citizens, personnel and property, including our Embassy, in South Sudan.”

After an aborted rescue mission of U.S. citizens Saturday, 380 U.S. officials and private citizens — as well as 300 citizens of other nations — were evacuated from South Sudan on chartered flights and military aircraft, according to a statement Sunday from State Department spokeswoman Jen Psaki.

Officials “took steps to ensure fighting factions were aware these flights were a humanitarian mission,” Psaki said.

The president’s Sunday missive notifying House and Senate leaders that U.S. forces are engaged abroad was standard operating procedure. White House officials said the notification should not be interpreted as an escalation of U.S. military operations in the region.

But Obama, who has just begun a two-week vacation with his family in Hawaii, appeared to be leaving his options open as he receives daily briefings from his national security advisers on the developing situation in South Sudan. Fighting erupted in that nation last week between supporters of President Salva Kiir and former Vice President Riek Machar.

The four servicemen were wounded Saturday when South Sudanese militias fired on three U.S. military aircraft that were en route to Bor, the capital of Jonglei state, where South Sudan’s army has been struggling for control with forces loyal to Machar.

In new details revealed in Obama’s letter to the House speaker and Senate leader, the president said about 45 additional U.S. military personnel were deployed Saturday on the mission to evacuate U.S. citizens from Bor. The rescue mission was aborted after the militia fired on U.S. planes.

Obama was told Saturday that the four U.S. service members hit by gunfire in the attack were all in stable condition. They were treated in Nairobi, Kenya, after their aircraft was diverted to Kampala, the Ugandan capital, according to the Associated Press.

As the army has lost control of key regions in South Sudan, the fledgling nation has spiraled toward civil war with the military divided and defectors taking control of key areas. Tribal rivalries between Kiir and Machar, whom Kiir dismissed in July, have fueled the crisis.

Kiir has accused Machar of attempting a coup. The former vice president denied the allegation, but there are reports that rebel factions in Jonglei and Unity states are under his command.