NAIROBI, Kenya — British, Canadian and Kenyan citizens are among 3,000 foreigners trapped in a South Sudan city experiencing bouts of heavy machine-gun fire, one of the most violent areas of a weeklong conflict that has likely killed more than 1,000 people, a top U.N. official said Monday.

Australians, Ugandans and Ethiopians are also among 17,000 people seeking protection at a U.N. base in Bor, a city that could see increased violence in coming days, said Toby Lanzer, the U.N.’s humanitarian coordinator.

Lanzer said there are no firm numbers available as to the death toll. The number of internal refugees is probably more than 100,000, said Lanzer, who is seeking urgent financial assistance from the U.S., Britain and other European countries.

“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 called for more peacekeepers Monday to strengthen the U.N. force, saying he is especially worried about ethnically targeted killings. He was sending his recommendation to the Security Council, which scheduled a meeting late Monday to discuss the request.

The U.N. has about 7,000 peacekeepers and international police in the world’s newest nation. One diplomat, speaking on condition of anonymity because he wasn’t authorized to disclose the information to the media, said he expects the secretary-general to request 5,000 additional troops.

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 would spend 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-gun fire.”

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.