It plans to seek funds next week to help more than a quarter of a million Kenyans forced to flee their homes, but said its appeal to donors could be expanded.

"We estimate that around 500,000 people will need aid in coming weeks and months," Elisabeth Byrs, spokeswoman of the U.N. Office for the Coordination of Humanitarian Affairs (OCHA), told a news briefing in Geneva.

"We fear malnutrition will rapidly worsen if insecurity and the lack of access to food and assistance persists."

Kenya's opposition said on Friday it would restart protests against President Mwai Kibaki's disputed re-election.

At least 500 people have already been killed in vicious ethnic clashes since the December 27 vote.

The U.N. World Food Programme (WFP) has distributed food for 33,000 people in Nairobi's slums, where many residents have lost their jobs and goods are scarce, spokeswoman Christiane Berthiaume said.

"The situation is calmer but very volatile because the political situation is not stable yet. We are just crossing our fingers and hoping it will remain calm," Berthiaume said.

Nearly one in three Kenyan children under five were already suffering severe malnutrition before the crisis, the U.N. Children's Fund (UNICEF) said.

UNICEF has distributed 11,000 family kits with tarpaulins, blankets and soap in Eldoret in the Rift Valley, which has been worst hit by violence.

The U.N. inter-agency appeal will be launched next week in New York.