The US military is redeploying aircraft and other forces in the Horn of Africa to prepare for possible further evacuations of Americans from South Sudan, the Pentagon said Monday.

"The secretary of defense has been following this situation very closely," spokesman Colonel Steven Warren told reporters.

"We are repositioning our forces in the area of concern," Warren added, saying units were readying in case the government orders more evacuations of Americans still on the ground.

Apart from flying out foreign nationals, no other military action was under consideration, he said.

About 100 Americans were evacuated by US forces on Sunday after a previous attempt on Saturday had to be called off when American aircraft came under fire from the ground, with four troops wounded.

About 30 Americans remain at the US embassy in Juba, where "non-emergency" staff were already flown out in recent days.

The Pentagon was looking to move cargo planes and helicopters to a US base in Djibouti, as well as a special Marine Corps unit, a defense official told AFP on condition of anonymity.

With 150 troops, the Marine Corps "special purpose air-ground task force" includes Osprey tilt-rotor aircraft, refueling tankers and infantry.

The military has already sent in a 47-member joint task force to Juba to reinforce security at the US embassy.

Washington has bolstered security at diplomatic outposts after an attack in September 2012 at a mission in Benghazi that killed the US ambassador to Libya, Chris Stevens, and three others.

President Barack Obama's administration came under criticism from Republicans in Congress for its response to the Benghazi attack and its explanation of the incident afterward.

On Saturday, four US tilt-rotor V-22 Ospreys flying to Bor for an evacuation had to turn back after coming under small arms fire.

The Ospreys were carrying Navy SEAL special operation forces for the evacuation mission, officials said.

Four service members on board were wounded and taken to Nairobi for medical treatment.

Three of the four wounded troops have been flown to an American base in Germany for further treatment but one of the troops was not "stable enough" and remains in Nairobi, according to Warren.

The Ospreys were not escorted by attack helicopters or fighter jets and it appeared US commanders assumed their forces could fly in without encountering hostile fire.

The United Nations had conducted its own helicopter flights without being attacked, officials said.

In a letter to members of Congress on Sunday, Obama said Saturday's operation involved 46 service members.

Obama has warned South Sudan that any military coup would mean an end to diplomatic and economic support from the United States and its allies.