South Sudan's military says rebels are moving toward a flashpoint town, as hopes fade that a cease-fire deadline will be met.
An army spokesman told reporters Monday that rebel troops are advancing on the Jonglei state capital of Bor.
Reuters news agency quotes Aguer as saying that shootings have taken place just outside the town.
A spokesman for the U.N. mission in South Sudan told VOA that the United Nations is extremely concerned about reports of large numbers of armed youth advancing toward Bor.
"We see their reported advance in the general direction of Bor as a very troubling development. Armed youths of various ethnic backgrounds in Jonglei state have been at the center of much of the intercommunal fighting that has plagued South Sudan since the country became independent two and-a-half years ago.
Earlier Monday, Ugandan President Yoweri Museveni said East African nations have warned South Sudanese rebel leader Riek Machar to sign a cease-fire deal or face action by regional nations.
Mr. Museveni, who met South Sudanese President Salva Kiir in South Sudan's capital, Juba, Monday, said Machar has been given four days to respond to the offer.
"We gave Riek Machar some four days to respond and if he doesn't we shall have to go for him, all of . That is what we agreed in Nairobi.''
Regional leaders are pressing for face-to-face talks between Mr. Kiir and Machar, his deposed vice president, by December 31.
Uganda says it has troops stationed at Juba's international airport tasked with "facilitating evacuation of civilians," but United Nations workers in the city say the forces are more widely deployed. Mr. Museveni and Mr. Kiir are strong allies.
The United Nations says U.N. Secretary-General Ban Ki-moon called Mr. Kiir Monday and welcomed the president's commitment to a cease-fire. He also urged Mr. Kiir to consider the early release of political prisoners.
The medical aid group Doctors Without Borders said Monday tens of thousands of South Sudanese have fled Bor since violence broke out there last week between government troops and the Machar-backed force - the so called "White Army."
White Army youths are known for the white powder they use to cover their skin as an insect repellant. Like Machar, they are ethnic Nuers while President Kiir and his loyalists are ethnic Dinka.
The tribal violence erupted earlier this month, when the president accused Machar of attempting a coup. The United Nations says the fighting has left more than 1,000 people dead and displaced tens of thousands.