Story highlights African Union calls for an immediate truce

Wounded service members in stable condition in Kenya, military official says

U.S. troops were on mission to evacuate Americans from violence-hit city

Pentagon trying to determine how to evacuate more than 30 Americans

A mission to evacuate Americans from South Sudan was aborted Saturday when an aircraft carrying U.S. military members was fired upon as it prepared to land in Bor, wounding four of them, the Pentagon said.

The most severely damaged aircraft was thought to have been hit in the fuel line, a military official speaking on condition of anonymity said.

All three aircraft -- CV-22 Ospreys -- were diverted to Entebbe, Uganda, which is not where their flights originated, the official said. Another aircraft then flew the wounded to Nairobi, Kenya, U.S. Africa Command said in a statement.

The four service members were in stable condition after treatment, the statement said.

Pentagon officials were trying to determine how to mount another effort to evacuate the roughly three dozen Americans in South Sudan, where they have been working for the United Nations, a senior U.S. official said on condition of anonymity.

Secretary of Defense Chuck Hagel was "reviewing options," Pentagon press secretary Rear Adm. John Kirby said. "Whatever we do it will be in coordination with the State Department," he added.

The White House said U.S. President Barack Obama was briefed before dawn Saturday while aboard Air Force One after landing in Hawaii, then met with his national security team on the matter.

The fighting has displaced as many as 100,000 people, many of whom have crossed the Nile River, he said, adding that he feared a humanitarian disaster was unfolding.

South Sudanese President Salva Kiir blamed soldiers loyal to his former vice president, Riek Machar, for starting this month's violence.

Tensions have been high in South Sudan since July, when Kiir dismissed Machar and the rest of the Cabinet. The move inflamed tensions between Kiir's Dinka community and Machar's Nuer community.

Casualties include soldiers and number in the hundreds, the government said.

U.S. Secretary of State John Kerry called Kiir on Saturday and discussed ways to halt the violence. It was Kerry's second call to Kiir since Thursday night.

"Secretary Kerry emphasized that only through leadership and political dialogue will the challenges facing South Sudan be resolved," State Department spokeswoman Jen Psaki said in a statement.

The two men "discussed the need to prevent ethnic violence, their concern for the welfare of thousands of internally displaced persons fleeing the conflict, as well as for the safety of U.S. citizens in South Sudan, and they agreed to speak again soon," she said.

Meanwhile, the State Department issued an emergency message for U.S. citizens, calling on them to avoid the area around the airport in Bor. Limited flights were continuing from Juba International Airport.

On Friday, Kerry said he was sending a special envoy -- Ambassador Donald Booth -- to the country.

"Now is the time for South Sudan's leaders to rein in armed groups under their control, immediately cease attacks on civilians, and end the chain of retributive violence between different ethnic and political groups," Kerry said in a statement.

Saturday's violence wasn't the first this week to harm foreign troops in South Sudan. On Thursday, attackers killed two Indian army peacekeepers, wounded a third, and killed 20 of 30 civilians who were seeking refuge at the United Nations' Akobo base, the U.N. said.

In a news release, the African Union called for an immediate truce.

It said that the chairwoman of the AU Commission, Nkosazana Dlamini-Zuma, was "profoundly dismayed at the recent turn of events" and condemned the killing of innocent civilians and U.N. peacekeepers in Bor as a war crime.

South Sudan became the world's newest country when it split from Sudan in July 2011. The split happened after a 2005 peace agreement ended years of civil war between the largely Animist and Christian south and the Muslim-dominated north.

The deal led to a January 2011 referendum in which people of the south voted to secede from Sudan.