Armed rebels were said to be in control of some of South Sudan's oil fields Friday, raising questions of how long the country's oil will flow and whether Sudan could enter the conflict which showed no signs of ending.

President Salva Kiir implored his country to turn away from ethnic violence and met Friday with foreign ministers from neighboring states, including Kenya and Ethiopia, who flew into Juba, the capital, to help calm tensions after a week of ethnic strife that is estimated to have killed hundreds.

Kiir did not speak publicly, but the government's Twitter feed attributed this quote to him: "Those who may want to take the law into their hands, the long arm of the government will get them."

On Friday evening, U.S. Secretary of State John Kerry announced Special Envoy for Sudan and South Sudan, Ambassador Donald Booth would be traveling to the region immediately.

"Now is the time for South Sudan's leaders to rein in armed groups under their control, immediately cease attacks on civilians, and end the chain of retributive violence between different ethnic and political groups," Kerry said in a statement.

The U.N. Security Council expressed "grave alarm" at the rapidly deteriorating security crisis, condemned targeted ethnic violence and demanded an end to the fighting.

The U.N.'s most powerful body urged the president and ousted vice president Riek Machar "to demonstrate leadership in bringing a swift and peaceful resolution to this crisis."

France's U.N. Ambassador, Gerard Araud, the current council president, told reporters that Kiir and the widow of South Sudan's rebel hero John Garang, who led the country's fight for independence, have agreed to enter an unconditional dialogue.

There was no word yet from Machar, who is believed to be in hiding.

Kiir, an ethnic Dinka, earlier this week said an attempted coup had triggered the violence, and the blame was placed on Machar, an ethnic Nuer. But officials have since said a fight between Dinka and Nuer members of the presidential guard triggered the fighting Sunday night that has since spread across the country. Machar's ouster from the country's No. 2 political position earlier this year had stoked ethnic tensions.

The Security Council said the violence resulted from a "political dispute among the country's political leaders" that could affect not only South Sudan, but neighboring countries and the entire region.

"The political crisis could lead to a general and political civil war if we do not solve it very quickly through dialogue," Araud warned.

Fighting continued to spread on Friday in Jonglei and Unity state, an oil area, as armed groups opposed to the nation's military emerged, said a South Sudan expert communicating with combatants and U.N. officials in strife-torn regions outside the capital.

"We've seen the conflict expand quite rapidly and quite dramatically in recent days. We've seen the emergence of different armed groups under different commands, and we've seen the former vice president say he's not interested in talks that don't end in Salva Kiir stepping down," said Casie Copeland, South Sudan analyst for the International Crisis Group. She added that the arrival of regional foreign ministers in Juba "is genuinely a good thing."

The U.N. peacekeeping mission strongly condemned the unprovoked attack on a U.N. base in Akobo in Jonglei state, near the Ethiopian border, on Thursday that killed two Indian peacekeepers, injured a third, and also killed at least 11 civilians who sought refuge there.

The mission said an estimated 2,000 armed youths, believed to be Nuer, surrounded the base and opened fire in the direction of Dinkas who earlier had sought refuge inside the compound. While trying to open negotiations with the assailants, peacekeepers inside the compound came under sustained attack, it said.

South Sudan's government admitted hursday that the central government had lost control of Bor, the capital of Jonglei which is the country's largest and most populous state.

Araud said Friday that 14,000 civilians have taken refuge in a U.N. camp in Bor, protected by a company of Indian peacekeepers.

There was heavy fighting in the city, he said, and some firing around the U.N. camp.

Up to 3,000 people with weapons were close to the camp and there were fears of a repetition of the attack carried out on Akobo, Araud said.

He said oil facilities have been raided "and apparently the Dinka employees have been killed." He also reported rising tensions around Bentiu, the capital of Unity, "which are linked to the oil installations."

South Sudan's oil fields have historically been a target for rebel movements and Copeland said armed opposition groups appeared to be in control of some oil fields in Unity state.

"The potential for oil wealth to exacerbate the current power struggle should not be underestimated," said Emma Vickers of Global Witness, a London-based group that investigates and campaigns to prevent natural resource related conflict. "If rebel forces were to capture the oil fields, they could effectively hold the government to ransom."

The United Nations said Friday that 35,000 people continue to seek refuge at U.N. bases in three locations across the country, including 20,000 at two bases in the capital. Several hundred people were also seeking shelter in Bentiu.

The U.S. Embassy had a fifth emergency evacuation flight on Friday to move Americans out of the country. British, German and Dutch planes were also scheduled to fly out. Hundreds of foreigners, including aid workers, have hurriedly left South Sudan this week at the urging of foreign embassies concerned about the possibility of out-of-control violence.

Forty-five U.S. troops were dispatched to Juba earlier this week to protect U.S. citizens and property.

A top U.N. official in the country, Toby Lanzer, said Friday that "difficulties" continued in Jonglei including Bor, where a top military commander loyal to Machar defected from the army this week, taking his troops with him.

South Sudan gets nearly 99 percent of its government budget from oil revenues. The country reportedly earned $1.3 billion in oil sales in just five months this year, Global Witness said.

South Sudan, the world's newest country, peacefully broke away from Sudan in 2011 after decades of war and years of negotiations that former U.S. President George W. Bush invested heavily in. The south's oil flows north through Sudan's pipelines, and a rebel takeover of southern oil fields could invite Sudan into the conflict. .

After meeting with Kiir, Ethiopia's Foreign Minister Tedros Adhanom Ghebreyesus said the ministers from a regional bloc known as IGAD were in the country to "understand the situation first hand." He called the meeting with Kiir productive but did not provide details.