JUBA, South Sudan - South Sudanese President Salva Kiir on Wednesday called for an end to ethnically based attacks on civilians, even as fighting raged in a key oil-producing region and a fast-spreading humanitarian crisis worsened.

"Whoever is still doing whatever malicious things in the residential areas, they must be told to stop," Kiir told congregants at Mass in Juba's largest cathedral, St. Theresa's. "That is not supporting me. Instead you are destroying me."

His appeal comes amid wide reports of extrajudicial killings, rapes, beatings, and mass graves, according to victims, witnesses, and U.N. human-rights officials, raising fears of a civil war.

On Wednesday, the U.N. mission in South Sudan denied a report of a mass grave that was issued by the office of a U.N. Commissioner for Human Rights. The mission said the report was an inflation of a "skirmish" in which 15 people were killed.

'Ruined'

Ethnic Dinka government soldiers are accused of killing ethnic Nuer civilians in Juba, the capital, while Nuer gangs have targeted Dinka civilians in the towns of Bor, Bentiu, and elsewhere in this fragile oil-producing nation, which gained independence from Sudan in 2011.

Kiir, without his trademark black cowboy hat, first addressed the crowd in English. Switching to Arabic, he later said: "This is our third Christmas in an independent state, and they ruined this Christmas for us. . . . Tribal fighting will lead us nowhere."

Even as Kiir spoke, government soldiers were fighting troops loyal to former Vice President Riek Machar in Malakal, the capital of the oil-rich state of Upper Nile, said South Sudanese military officials. The clashes have continued despite efforts by the United States, as well as European and African nations, to mediate an end to the violence.

Quickly spreading

The crisis erupted Dec. 15 when soldiers loyal to Machar, who is a Nuer, clashed with troops loyal to Kiir, who is a Dinka, in Juba. That prompted Kiir to accuse Machar of attempting a coup. Since then, the violence has quickly spread to half of South Sudan's 10 states.

On Wednesday, the United Nations announced that the number of people who have sought shelter at U.N. peacekeeping bases nationwide had mushroomed to 58,000. More than 92,000 have fled their homes.

In a tweet Wednesday, Toby Lanzer, deputy special representative to the U.N. mission in South Sudan, wrote that aid groups need $166 million immediately "to provide the best protection and aid to civilians struck by evolving crisis."

This article contains information from the Associated Press.