Australians are among 17,000 people seeking sanctuary at a UN base in the South Sudan city of Bor, as violence intensifies across the country.

UN humanitarian co-ordinator Toby Lanzer said as well as those looking for UN protection in Bor, which may see increasing violence over coming days, there are also 3,000 people, including British, Canadian and Kenyan citizens, trapped in a city already experiencing heavy bouts of machine gun fire. More than 20,000 civilians have sought safety in two UN camps in the capital city of Juba.

The week-long conflict in South Sudan, a country that has only existed since 2011, has killed more than 1,000 people, it is estimated. Some aid workers put the figure in the tens of thousands. There are reports of targeted massacres, ransackings and destruction of homes and businesses.

The number of internal refugees is probably more than 100,000, said Lanzer, who is seeking urgent financial assistance from the US, 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.”

The Australian Department of Foreign Affairs and Trade (Dfat) told Guardian Australia it was aware of a number of Australians still in South Sudan but had not received reports of anyone injured in the violence.

“The security situation in South Sudan remains exceptionally volatile and we strongly urge Australians in South Sudan to leave immediately while commercial options are available,” said a Dfat spokeswoman.

“If it is not safe to depart, Australians in the country should take all precautions and stay indoors in a safe location with sufficient stocks of food and water.”

The UN secretary-general, Ban Ki-moon, urged the Security Council late on Monday to add 5,500 troops and police to the 7,000-strong UN 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 Associated Press, that the troops be transferred from UN 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 UN ambassador, Gerard Araud, the current council president, said he expects the council to vote Tuesday on a resolution authorising the transfers.

The secretary-general called on member states to urgently provide transport to get troops, police and equipment to South Sudan.

He said the UN 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 UN 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 US military aircraft on Saturday, forcing the Osprey advanced helicopter-airplane hybrids to abort their evacuation mission. On Sunday, the US evacuated Americans by civilian US and UN helicopters.

The US 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 15 December. 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. Aguer said 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 of 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 UN is fortifying its position there.

The US over the weekend deployed about 46 troops to help evacuate American citizens. That was in addition to 45 troops sent to Juba last week to protect the US embassy. Four US troops were wounded in the evacuation attempt Saturday.

President Obama over the weekend sent a letter to congressional leaders letting them know he may take further military action in South Sudan to protect US 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 US envoy is also headed toward the region. South Sudan experienced decades of war with Sudan, which it peacefully broke away from in 2011.