UNITED NATIONS, June 21 (Reuters) - One medical charity has treated 500 victims of sexual violence in Darfur in four months and this is just a fraction of such attacks in the Sudanese province, a senior U.N. official said on Tuesday.

Under-Secretary General Jan Egeland told the Security Council women and children were being systematically raped and assaulted in the ravaged region and urged Sudanese authorities to do more to protect civilians and end a culture of impunity.

Addressing the U.N. Security Council on the need for more international effort to protect civilians in armed conflicts, Egeland said Sudan and the Democratic Republic of Congo were among the countries where sexual violence was worst.

The Darfur conflict broke out two years ago when rebels took up arms against the Sudanese government, complaining of discrimination. Khartoum is accused of retaliating by arming militias who burned villages and killed and raped civilians.

At least 180,000 people have died from violence, hunger and disease and two million have been driven from their homes.

Egeland said medical charity Medicins Sans Frontieres had reported treating 500 survivors of sexual violence in Darfur in just four months.

"We believe this represents only a fraction of the total victims," he said, adding that the impact of the violence was compounded by Sudan's failure to acknowledge the scale of the problem and to act to stop it.

"Not only do the Sudanese authorities fail to provide effective physical protection, they inhibit access to treatment." He said in some cases unmarried women who became pregnant after being raped had been treated as criminals and subjected to further brutal treatment by police.

"This is an affront to all humanity," Egeland said.

The U.N. Security Council has asked the International Criminal Court to investigate alleged war crimes in Darfur. Earlier this month Sudan formed a special court to try alleged criminals in the western Darfur region.

RAPE AS WEAPON

"In Darfur ... rape is systematically used as a weapon of warfare," Egeland said.

France had organized the debate on civilian casualties around the world, an issue which has been before the council for several years and eventually resulted in a mandate for peacekeepers to protect civilians.

Egeland said that while he was concerned about the targeting of civilians in conflicts around the world, including Iraq where he said as many as 1,000 civilians may have been killed since April, his biggest concern was Africa.

"In North Kivu, in eastern Congo, one nongovernmental organization reported 2,000 cases of sexual abuse ... in one month," Egeland said. He said most of the cases were rape.

He said U.N. officials in the area estimated there were at least 25,000 cases a year of sexual violence against women and children in North Kivu, a situation partly attributable to the breakdown of discipline in the regular armed forces.

The United Nations has more than 16,000 peacekeepers in the Congo, where peace deals in 2003 officially ended a five-year war that killed nearly four million people, mostly from hunger and disease. Armed groups still operate in much of the east.

He picked out Ivory Coast, Liberia, northern Uganda and Nepal as areas where civilians were most endangered by conflicts.