The US military on Monday deployed a special Marine Corps unit and aircraft to the Horn of Africa to prepare for possible further evacuations of Americans from South Sudan, officials said.

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

"We are repositioning our forces in the area of concern," Warren said.

The troops would be ready, if ordered, for "evacuations and the security associated with an evacuation."

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

The Pentagon redeployed cargo planes, helicopters and 150 members of a special Marine Corps unit to Djibouti, the site of a major US base, two defense officials told AFP on condition of anonymity.

The Marine Corps "special purpose air-ground task force" includes Osprey tilt-rotor aircraft, C-130 cargo planes, aerial refueling tankers and infantry.

The task force, which has a total of 500 troops, is currently stationed at Moron air base in southern Spain.

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

About 100 Americans were evacuated 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 amid rising violence and with South Sudan sliding towards civil war.

UN Secretary General Ban Ki-Moon meanwhile said Monday he would urge the Security Council to reinforce the United Nations mission in South Sudan with additional troops to protect civilians.

The UN mission in the country is currently capped at 7,000 soldiers and 900 police. The mission includes more than 2,000 civilians.

A senior US administration official told reporters Washington was weighing the UN request for assistance.

Washington created the special Marine crisis-response unit and bolstered security at diplomatic outposts after a September 2012 attack 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, which can take off like a helicopter and fly like a plane, 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.