The head of the United Nations called Tuesday for an immediate cease-fire in eastern Congo to urgently help at least 100,000 refugees cut off by fighting in rebel-held areas.

"Because of the ongoing fighting, these people have received virtually no assistance. Their situation has grown increasingly desperate," said U.N. Secretary General Ban Ki-moon.

He said for at least 100,000 refugees trapped behind rebel lines around the towns of Rutshuru and East Masisi the "lifeline has been cut off," and their situation is "very serious and dire."

"I urgently call for an immediate cease-fire in these areas to allow humanitarian assistance to reach many thousands of displaced persons," he said.

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About 250,000 people have been displaced by fighting in eastern Congo, where rebels led by Laurent Nkunda are battling Congolese forces. The conflict is fueled by festering ethnic hatred left over from the 1994 slaughter of a half-million Tutsis in Rwanda, and Congo's civil wars from 1996-2002, which drew neighboring countries in a rush to plunder Congo's mineral wealth.

The secretary-general said he was mobilizing all U.N. humanitarian agencies "to find necessary humanitarian assistance" to help those who have received no aid.

Ban returned from a summit in Nairobi last week that had called on all armed groups in the North Kivu province to observe an immediate cease-fire. Since the Nairobi meeting, Ban said he had met with representatives of the U.S., France, Britain, Netherlands, Canada, Japan, African Union and European Union.

A U.N. mission has been investigating reports of massacres. It visited 11 burial sites that witnesses said contained 26 bodies of combatants and civilians, Ban's spokeswoman, Michele Montas, said.

Ban said that "despite the Nairobi declaration, there are continued reports of sporadic fighting." He said he was very concerned about reports of targeted killings of civilians, looting, rape and the use of child soldiers.

U.N. agencies have been delivering food, medicine, fresh water and sanitation supplies but only to "areas where they can operate, most particularly in Goma," Ban said.

Ban said he would discuss with representatives of humanitarian agencies how they could gain access to the towns north of Goma.

He said about 3,000 more U.N. peacekeeping soldiers and police are urgently needed. Those would bolster the 17,000-strong U.N. force in Congo that has done little to prevent the rebels' advance, enraging the population.

The U.N. Security Council planned to take up Ban's request later Tuesday.