WASHINGTON (REUTERS) - The United States (US) military on Monday deployed about 150 Marines to a base in the Horn of Africa to prepare for possible further evacuations of American citizens from the deepening conflict in South Sudan, US officials said on Monday.

The deployment of a special crisis-response team of Marines, who are normally stationed at Moron Air Base in Spain, follows a thwarted evacuation attempt in South Sudan over the weekend in which four US soldiers were wounded by gunfire.

Three US officials, speaking to Reuters on condition of anonymity, said the Marines were sent to a base in Djibouti, a move that would allow them to deploy to South Sudan more quickly, if asked.

Colonel Steve Warren, a Pentagon spokesman, said earlier in the day that the US repositioning of troops would allow for possible "evacuations and the security associated with an evacuation", should the State Department make such a request.

South Sudan's government said on Monday it would start a major offensive to retake two strategic towns controlled by rebels loyal to former Vice-President Riek Machar.

On Saturday, four US service members were wounded when their military aircraft were hit by ground fire as they attempted to evacuate US citizens from the South Sudanese town of Bor. US citizens and other foreigners were later evacuated from the same area a day later.

So far, about 380 US officials and private US citizens have been evacuated from South Sudan, according to the State Department. It is unclear how many US citizens still are left in the country because many do not register with the US Embassy when they enter the country.

President Barack Obama told Congress in a weekend letter that "approximately 46 additional US military personnel" had been deployed by aircraft on Saturday to evacuate Americans.

Mr Warren said three of the US injured troops, all of whom took gunshot fire to their lower extremities during the attempted evacuation, were expected to be evacuated from Nairobi, where they have been treated, to a US military medical center in Germany later on Monday.

The fourth service member was not yet stable enough to be moved, Mr Warren said. He was being cared for by a specialised team that arrived in Nairobi over the weekend.

"We expect he'll be evacuated as soon as his condition stabilises," Mr Warren said.