In this photo released by the United Nations Mission in South Sudan (UNMISS), a U.N. helicopter transporting wounded civilians from Bor, the capital of Jonglei state and said to be the scene of fierce clashes between government troops and rebels, arrives at the airport in Juba, South Sudan, Sunday, Dec. 22, 2013. (AP Photo/UNMISS)
(CNN) -- U.S. Marines stood by to help evacuate Americans in South Sudan as the top U.N. official there warned Tuesday of a "breakdown in respect for the most basic rights of people" amid the country's widening military and humanitarian crisis.
U.N. Assistant Secretary-General Toby Lanzer tweeted that more accounts were reaching him of human rights abuses amid widening violence that has stoked fears of an all-out civil war in the world's newest country.
In Geneva, Switzerland, U.N. High Commissioner for Human Rights Navi Pillay called human rights abuses in the country a "serious and growing" problem.
"Mass extrajudicial killings, the targeting of individuals on the basis of their ethnicity and arbitrary detentions have been documented in recent days," Pillay said, according to the statement. "We have discovered a mass grave in Bentiu, in Unity State, and there are reportedly at least two other mass graves in Juba."
One U.N. official saw 14 bodies at the mass grave in Bentiu and another 20 on a nearby riverbank, said Ravina Shamdasani, a spokeswoman for the commissioner.
"As for the other two reported graves in Juba, we are still working to verify but it is very difficult, and there are reports that some bodies may have already been burned," she said.
South Sudan's breathtaking descent into widespread conflict comes a little more than two years after the nation was ushered into existence with help from international powers after decades of civil war between separatists in the oil-rich south and Sudan's northern government.
Fighting began midmonth after President Salva Kiir said forces loyal to the country's dismissed vice president, Riek Machar, launched a coup attempt. Kiir and Machar are longtime rivals.
In an interview with CNN, Machar denied there was a coup attempt.
South Sudan has suffered from sporadic violence since its formation in 2011.
But the broad nature of this conflict and the intensity of the violence -- which U.N. officials say has taken on ethnic overtones -- have raised fears of another genocide along the lines of Rwanda, the African nation where 800,000 people were slaughtered in 1994, according to the United Nations.
As the U.N. Security Council prepared to meet to discuss Secretary-General Ban Ki-moon's call to reinforce the agency's peacekeeping force in South Sudan, about 150 U.S. Marines waited in Djibouti, hundreds of miles east of South Sudan.
Marines on standby
They flew in from a base in Spain to await possible orders to enter South Sudan and help evacuate Americans who remained behind after a U.S.-led evacuation transported more than 300 U.S. citizens out of the country.
In a statement, the U.S. military's Africa Command said Monday it was positioning the Marines to be able to respond should conditions deteriorate even more. The decision grew out of the U.S. experience in Benghazi, Libya, where an attack last year killed U.S. Ambassador Christopher Stevens and three other Americans. No U.S. forces were close enough to respond quickly.
"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. These precautionary movements will allow us to do just that," the command said in its statement.
According to a senior administration official, 380 Americans and about another 300 third-country nationals have been evacuated.
"Based on registration, there are American citizens in other towns and areas throughout South Sudan. We are trying to track down how many may still be there. Many may have gotten out on their own. We are trying to track that down," the official said.
On Sunday, all Americans who showed up at the U.N. camp in the flashpoint town of Bor were evacuated safely, the State Department said. A State Department official said about 15 Americans were flown out Sunday. U.N. civilian staff were moved from a compound in Bor to Juba, the capital, on Saturday.
"The U.S. government is doing everything possible to ensure the safety and security of United States citizens in South Sudan," State Department spokeswoman Jen Psaki said in a statement on Sunday. "We are working with our allies around the world to connect with and evacuate U.S. citizens as quickly and safely as possible."
Sudanese forces retook Bor after a nearly six-hour fight Tuesday, Defense Minister Kuol Manyang Juuk said.
During his Christimas address to the public, President Kiir confirmed that the military had reclaimed Bor.
On the topic of the deadly violence in the country, Kiir called for calm and for unity.
The South Sudanese President expressed willingness for a peaceful settlement to differences with his former vice president and all other armed groups who have taken up arms against the state.
Sudanese military spokesman Col. Philip Aguer also predicted Tuesday that government forces would soon recapture the oil-rich Unity state.
Government forces and opposition fighters also clashed Tuesday in Malakal, he said.
The United Nations said fighting had spread to five of South Sudan's 10 states.
It described the situation in Juba as "stable but tense," but said conditions in Jonglei were deteriorating, with reports of clashes between armed factions near Bor, the site of a U.N. peacekeeping base. Conditions were also tense in Bentiu, where the United Nations reported military units on the move amid sporadic fighting.
The United Nations also said it had credible reports civilians were being attacked and killed based on their ethnicity, and reported that private property and the facilities of some humanitarian groups were being looted.
Meanwhile, U.N. officials struggled to make accommodations for some 45,000 people crowding its compounds seeking shelter from the violence. Nearly as many people were seeking shelter elsewhere.
U.N. officials said they have concerns about health conditions among those fleeing the fighting and urged assistance for medical care and other needs.
The U.N.'s Lanzer, who visited the base in Bor on Monday, said in a statement he had heard "heartbreaking accounts of people's suffering."
Ban, who is seeking to increase the 6,800-strong U.N. force in South Sudan by another 5,500, said Monday the agency will do its utmost to protect civilians.
"The U.N. stood with you on your road to independence," Ban said in a message to the people of South Sudan. "We will stay with you now. I know that the current situation is causing great and growing fear. You are seeing people leave the country amid increasing chaos. The U.N. will stay with you."
Secretary of State John Kerry has spoken with Machar in a bid to get talks started between the former vice president and the government, a senior State Department official told CNN.
"He urged him to implement a cease-fire and begin mediated political talks," the official said.
The European Union's foreign affairs minister, Catherine Ashton, said the violence threatens to send South Sudan "spiraling into a disaster for both its own people and the region."
"Such a situation can, and must be avoided," she said in a statement.
As Ban did Monday, she called for all sides to begin talks to resolve their differences.
"This dialogue must include all groups, including those whose leaders are currently imprisoned and those in revolt against the government," she said. "The use of force will achieve nothing, and all must use their influence to immediately cease hostilities and prevent further fighting which will only cause the further shedding of innocent blood."
CNN's Khushbu Shah, Jennifer Rizzo, Marie-Louise Gumuchian, Barbara Starr and Antonia Mortensen and journalist Mading Ngor in Juba, South Sudan, contributed to this report.