About 150 U.S. Marines, along with transport and refueling aircraft, arrived Monday in Djibouti on the Horn of Africa to aid in possible evacuation and protection missions in strife-torn South Sudan, the Pentagon said.

“One of the lessons learned from the tragic events in Benghazi was that we needed to be better postured in order to respond to developing or crisis situations, if needed,” said Rear Adm. John F. Kirby, the Pentagon press secretary.

The Defense Department was criticized by many in Congress for its response time after last year’s terrorist attack in Benghazi, Libya, which left a U.S. ambassador and three other Americans dead.

The newly deployed Marines are part of a Marine Air-Ground Task Force based in Morón, Spain. Defense officials emphasized that they have not yet been ordered to South Sudan but were on standby in case they need to be deployed.

Four U.S. troops were wounded Saturday when an air mission to evacuate American citizens was fired upon in the city of Bor. That mission was aborted, leading to a letter to Congress on Sunday from President Obama saying that he “may take further action to support the security of U.S. citizens, personnel and property, including our Embassy, in South Sudan.”

The arrival of the Marines on the African continent coincided with a decision by U.N. Secretary General Ban Ki-moon to ask members of the United Nations for additional troops to bolster a 7,000-strong peacekeeping mission in South Sudan.

“The situation is of mounting urgency,” Ban said at U.N. headquarters in New York on Monday morning. “I am especially worried by reports of ethnically targeted killings.” He is expected to ask for as many as 5,000 additional troops in a request that the U.N. Security Council is likely to consider Tuesday.

The peacekeeping mission is one of three U.N. forces in South Sudan and its neighbor, Sudan, from which it became independent in 2011. Nearly 20,000 U.N. and African Union troops are in violence-plagued Darfur, a region on Sudan’s western border. A 4,000-strong U.N. Interim Security Force is based in Abyei, the oil-rich territory along the border of the two countries.

Although fighting between the north and the south has gone on for years, and led to a U.S.-backed peace treaty that in 2005 ended Africa’s longest-running civil war, the outbreak of internal hostilities in South Sudan has disrupted one of the largest U.S. aid programs in Africa. The United States has long been deeply engaged in the region, supporting the separation of the two states with massive attention and assistance.

The Obama administration requested nearly $400 million in assistance for humanitarian and governance programs in South Sudan in its 2014 budget.

Fighting that broke out more than a week ago has pitted South Sudanese troops loyal to President Salva Kiir against followers of his former deputy, Riek Machar, who was fired in July.

Thousands of civilians, including an unknown number of Americans and other expatriates, most of them working with aid organizations, have sought refuge inside U.N. bases that, in some cases, have been surrounded by hostile forces. About 380 Americans have been evacuated.

The Obama administration’s special envoy to Sudan and South Sudan, Donald Booth, on Monday visited Juba, the South Sudanese capital, where he said he had “a frank and open discussion with President Kiir.” The envoy said Kiir “committed to me that he was ready to begin talks with Machar to end the crisis without preconditions.”

In interviews with news organizations, Machar has made similar statements about participating in talks.

But even as Kiir promised a rapprochement, he told South Sudan’s parliament that government troops would be dispatched to retake Bor, the capital of central Jonglei state, which is under the control of Machar loyalists. The impending offensive threatens to worsen the political crisis and the humanitarian situation.

Machar loyalists also have seized control of Bentiu, the capital of the oil-producing Unity state. On Monday, clashes erupted in another oil-producing region, Upper Nile state, adding to the growing instability across the nation.

A senior U.N. official on Monday called upon international donors to continue providing money and resources to help save lives in a crisis that could grow to affect hundreds of thousands of people in the coming months.

“As the year comes to a close, South Sudan today, and for the past few days, has seen its greatest test since becoming an independent country and perhaps one of its greatest tests ever,” Toby Lanzer, the deputy special representative of the U.N. mission in the country, told reporters. “We are now faced with one of the largest emerging humanitarian crises, which comes on top of what was already a large humanitarian program here in South Sudan.”

Lanzer added that although the focus has been on Juba, Bor and Bentiu, he expects other parts of South Sudan affected by the crisis to need more humanitarian assistance.

“We are hearing harrowing tales from our colleagues who are hiding in the bush in different parts of the country, who are calling us and are saying that their communities are being affected and they need help,” he said.

Ragavan reported from Juba.