"What's happening in Kenya at the moment is a true humanitarian tragedy with a terrible impact on children, young women and women. The question of protecting victims is as important as the distribution of humanitarian aid," spokeswoman Veronique Taveau said.

Up to 100,000 children under the age of five are among the 250,000 people who have been uprooted by Kenya's post-election turmoil, the United Nations Children's Fund (UNICEF) said.

Many are staying in camps with poor hygiene and erratic distribution of clean water, where young girls and boys are easy prey for sexual violence which is on the increase, Taveau told a news briefing.

"The state has failed in its duty to protect the camps. It appears there is neither law nor order in numerous places and massacres with machetes are multiplying," she said.

Extreme ethnic violence had spread from Eldoret and Burnt Forest in the northern Rift Valley to Molo in the south, she said. It was increasingly difficult for UNICEF teams to reach people requiring aid urgently.

Around 850 people have died in violence since a December 27 election which re-installed President Mwai Kibaki in power. The opposition says the vote was rigged.

On Sunday, a group of men at a government-run camp in northern Rift Valley threatened 20 women who were telling UNICEF staff about recent rapes, Taveau said.

"The UNICEF team went to see the person responsible for security in the camp. The response was 'there is nothing going on in the camps, everything is fine'," she said, declining to identify the camp which holds some 100 displaced persons.

"It is the government's responsibility to protect this camp, not UNICEF's."

Other U.N. officials said security and humanitarian conditions in Kenya had deteriorated sharply since weekend clashes and house torchings in the towns of Naivasha and Nakuru which killed at least 30 people.

"We are concerned about the vicious circle of violence and reprisals which in turn create further waves of displaced persons," said Elisabeth Byrs, spokeswoman of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA).

Truck drivers hired by the World Food Programme have refused to leave the port of Mombasa due to insecurity, WFP spokeswoman Christiane Berthiaume said. The U.N. agency hoped a convoy could leave Nairobi under escort for the Rift Valley on Tuesday.

A convoy of three WFP vehicles was stoned on Monday and had to return to Eldoret. The drivers were unharmed, she said.