"I know there are many thousands of people seeking protection in churches," Lanzer said. "I know that we have our own staff that have literally walked into the bush and are communicating from there. That's where they say they are safest."
U.N. Secretary-General Ban Ki-moon urged the Security Council late Monday to add 5,500 troops and police to the 7,000-strong U.N. peacekeeping mission in South Sudan, citing growing violence in many parts of the country, human rights abuses, "and killings fueled by ethnic tensions."
Ban proposed in a letter to the council obtained by The Associated Press that the troops be transferred from U.N. missions in Congo, Darfur, Abyei, Ivory Coast and Liberia, along with three attack helicopters, three utility helicopters and a C130 military transport plane.
France's U.N. Ambassador Gerard Araud, the current council president, said he expects the council to vote Tuesday on a resolution authorizing the transfers.
The secretary-general called on member states to urgently provide transport to get the troops, police and equipment to South Sudan.
He said the U.N. mission's capacity to investigate human rights abuses is also being urgently strengthened.
Ban said the situation in South Sudan "is of mounting urgency," with tens of thousands of people displaced and about 45,000 seeking protection at U.N. bases.
He said he was spending most of Monday urging regional leaders to increase military support and "political backing for efforts to defuse the crisis."
In a message to the Sudanese people, the secretary-general said: "The United Nations will stay with you. We will do our utmost to protect you, to provide the humanitarian assistance you need, and most of all to help the country regain the path to peace."
Bor is the city where rebel forces fired on three U.S. military aircraft on Saturday, forcing the Ospreys advanced helicopter-airplane hybrids to abort their evacuation mission. On Sunday, the U.S. evacuated Americans by civilian U.S. and U.N. helicopters.
The U.S. over the last week has evacuated 380 Americans and 300 others from South Sudan, which has seen vicious, ethnically targeted violence pulse through the nation.
Lanzer, who spent the weekend in Bor, said the city is experiencing tense, sporadic clashes and "fairly consistent gunfire and heavy machine gunfire."
South Sudan forces are advancing toward Bor, the capital of Jonglei state, but have not yet confronted forces that defected and pledged allegiance to the former vice president, said South Sudan military spokesman Col. Philip Aguer.
The violence began late on Dec. 15. South Sudan President Salva Kiir, an ethnic Dinka, has said an attempted military coup triggered the violence, and the blame was placed on former Vice President Riek Machar, an ethnic Nuer. Other officials have since said a fight between Dinka and Nuer presidential guards triggered the fighting.
New fighting was reported Sunday night in Upper Nile, one of South Sudan's two oil-rich states. Warrior Security, a private company, reported on Monday the deaths of 20 people in Nasir County in a dispute involving Machar supporters. Doctors Without Borders said it received 24 gunshot victims at its medical facility.
Col. Philip Aguer, South Sudan's military spokesman, said elsewhere in the state that civilians who had been told their relatives were killed in Juba tried to mete out mob justice. Warrior Security said ethnic Dinkas were attacked and killed. Aguer said he did not have a death toll.
Analysts have suggested that a tribal militia known as the White Army from the Lou Nuer ethnic group is moving toward Bor, which is populated by Dinkas. Lanzer said he couldn't say anything with precision about those reports. Aguer said he has no confirmation on militia movements but that community leaders are trying to persuade the Lou Nuer not to become involved.
"Everybody knows that Bor is a strategic location," Lanzer said. "It would be difficult for me to imagine a scenario in which Bor is completely calm and safe over the coming days," he said, adding that he thinks violence could become "very heavy," the reason the U.N. is fortifying its position there.
The U.S. over the weekend deployed about 46 troops to help evacuate American citizens. That was in addition to 45 troops sent to the capital, Juba, last week to protect the U.S. Embassy. Four U.S. troops were wounded in the evacuation attempt Saturday.
Obama over the weekend sent a letter to congressional leaders letting them know he may take further military action in South Sudan to protect U.S. citizens, personnel and property.
The central government acknowledged on Sunday it has lost control of Bentiu, the capital of Unity, and the surrounding oil fields. Oil fields in the country's Upper Nile state are still controlled by the central government, Aguer said.
East African leaders are pushing diplomatic efforts to avoid a full-blown civil war. Obama's U.S. envoy is also headed toward the region. South Sudan experienced decades of war with Sudan, which it peacefully broke away from in 2011.
Associated Press Writer Edith M. Lederer contributed to this report from the United Nations.