Thousands of civilians have been killed or wounded and millions more displaced since fighting began in Sudan 100 days ago, UN humanitarian agencies said on Monday.

"One hundred days of fighting have exacted a devastating toll on the civilian population," the UN Office for the Coordination of Humanitarian Affairs (OCHA) said. "It is a senseless conflict, and it must stop now."

The UN Children's Fund (UNICEF) reported that during the same period, on average at least one child was injured or killed every hour of the conflict, OCHA said.

"That's based on credible reports that at least 435 children have been killed and more than 2,000 injured in the past 100 days," the office said. "As these are just the numbers reported to UNICEF sources, the true figure is likely to be far higher."

The humanitarian office said a report by the United Nations High Commissioner for Refugees (UNHCR) concentrated on the rapidly escalating numbers of displaced people in Sudan fleeing for safety.

The UNHCR also warned of a serious health and nutrition crisis unfolding in White Nile, where their teams on the ground said nearly 300 South Sudanese refugee children have died from suspected measles and malnutrition since the conflict began on April 15.

The World Food Program (WFP) has supported more than 1.4 million people across Sudan with food and nutrition assistance, hoping to scale up assistance to 5.9 million conflict-affected people in Sudan by the end of this year, the office said.

The WFP warns that over 40 percent of Sudan's population, more than 19 million people, face hunger due to the conflict. It is the highest number of hungry people ever recorded in the country, OCHA said.

"Sudan is one of the most complicated humanitarian responses in the world right now," OCHA said of the conflict brought on by fighting between opposing factions of Sudan's military. "Still, we and our humanitarian partners are doing everything we can to respond to the crisis, despite the continued fighting and access restrictions."