About 150 Marines have been dispatched to Djibouti in the Horn of Africa in case they're needed to help evacuate more Americans from South Sudan, U.S. military officials said today.

The move comes days after an unsuccessful attempt to evacuate 15 American civilians trapped in an eastern city in South Sudan. That mission was aborted when three aircraft sent to evacuate the Americans took heavy ground fire, injuring four aboard, officials said. The Americans were eventually evacuated Sunday aboard United Nations helicopters.

U.S. Marines arrived today in Djibouti, where there is a U.S. military base, along with several MV-22 Osprey aircraft and KC-130 cargo aircraft that can serve as refueling tankers, according to the U.S. Africa Command. They are part of a 500 Marine force based in Moron, Spain, that is available to Africom to respond to crises in northern and western Africa.

Last week, 45 members of a similar response force for eastern Africa arrived in South Sudan's capital city of Juba to help protect the U.S. embassy and its remaining staff.

Gen. Dave Rodriguez, the commander of Africom, is "repositioning forces in the region in an effort to give himself the maximum flexibility to respond to any follow-on requests from the Department of State," Pentagon spokesman Col. Steve Warren said today.

A Defense Department official said the Marine force would be available to assist with further evacuations of Americans from South Sudan if needed.

Violence has raged in South Sudan for the past week as military forces loyal to President Salva Kiir have battled military forces loyal to former Vice President Riek Machar. There are concerns that the violence could become a full-scale civil war based along ethnic lines.

U.N. officials believe more than 1,000 people have died in the fighting and they estimate that tens of thousands of civilians have sought refuge in its compounds throughout the country. A senior administration official said that the United States is currently reviewing how it can help with requests from the United Nations for assistance in South Sudan.

U.S. diplomatic efforts for a peaceful end to the violence in South Sudan showed signs of progress today.

After a day of meetings in Juba, Donald Booth, U.S. envoy for South Sudan, said he had received a commitment from Kiir "that he was ready to begin talks with Riek Machar to end the crisis, without preconditions, as soon as his counterpart is willing."

Machar has reportedly said he will agree to talks only if Kiir's government releases some of his political allies.

Meanwhile, more details emerged from Saturday's aborted raid in the eastern city of Bor that is now under the control of forces loyal to Machar.

A War Powers Act notification letter sent by President Obama to Congress revealed that 46 U.S. military personnel were aboard the three Osprey aircraft sent to Bor to evacuate the 15 American civilians.

The mission was aborted when the aircraft took heavy small arms fire, wounding four Navy SEALs aboard the aircraft, officials said. All four suffered injuries to their lower extremities.

Three of the four were transported today from Nairobi, Kenya, to Germany for treatment at the U.S. military hospital in Landstuhl, officials said. The fourth service member will remain in Nairobi until he is stable enough to be transported to Germany.

The 15 American civilians they were trying to evacuate were eventually airlifted out of Bor on Sunday aboard United Nations helicopters.

A senior administration official said the decision to use the U.S. military to evacuate them out of Bor had been made several days ago when the United Nations did not have the capability of airlifting out of Bor.

The State Department said that it has evacuated about 380 Americans and 300 third-party nationals out of South Sudan. Officials have been wary of saying precisely how many Americans remain in South Sudan because not all of them had registered with the U.S. embassy in Juba and some may have made their own way out of the country.

South Sudan broke off from Sudan in 2011 and is described as the world's newest state. It has received more than $1 billion worth of American aid since then to help develop the infrastructure the new nation needs.

When asked if the United States had warned Kiir's government that it might cut off that aid if the violence continues, the senior administration official said the U.S. is weighing "all of our options as to how we will ensure that parties come to the table."

The official emphasized that no decision about the assistance has been made by the United States.