United Nations Secretary-General Ban Ki-moon is expressing alarm at the rising number of people being killed or forced from their homes by the fighting in South Sudan.
In a statement late Tuesday, Ban's spokesman said the U.N. chief also condemned both government troops and rebels for taking vehicles and food stocks from humanitarian groups.
The U.N. announced on Tuesday that the number of people displaced by a month of fighting has reached nearly 500,000, including almost 7,500 people who have fled to Uganda, Sudan, Kenya and Ethiopia.
Many civilians have sought refuge from the fighting at U.N. bases in South Sudan. In New York, U.N. spokesman Martin Nesirky said the number of people at the U.N. base in the northern town of Malakal has doubled in recent days to 20,000.
Rebels announced on Tuesday that they had captured Malakal, the capital of Upper Nile state.
Government spokesman Michael Makuei emphatically denied the report.
"This is not correct, this is baseless, this is unfounded. What happened in Malakal is that they, the rebels, attempted to attack Malakal and they have been repulsed, and some are still on the run up until now," said Makuei.
Makuei spoke to reporters in Ethiopia's capital, Addis Ababa, where the government and rebels continue talks on a possible cease-fire.
The government spokesman said the talks are "progressing well" and said the two sides might sign a cessation-of-hostilities deal by Wednesday.
However, a member of the rebel delegation said that there are other issues that must be worked out prior to any agreement, including the role of Ugandan forces in South Sudan. The rebels have accused Uganda of assisting South Sudanese President Salva Kiir.
A proposed cease-fire has been held up by the government's refusal to release 11 political detainees, as demanded by the rebels.
Army forces loyal to President Kiir are battling soldiers who back former vice president Riek Machar. Kiir fired Machar in July.
The fighting in South Sudan has sparked fears of a full-fledged civil war in the country, which became independent from Sudan less than three years ago.