At least 180,000 people have been displaced by unrest as the humanitarian crisis grows after last week's disputed election in Kenya, say UN officials.
Some have been housed in makeshift camps while others have sought refuge in police stations or churches, fleeing violence that has claimed 350 lives.
In badly-affected western Kenya nearly all the refugees are hungry, and several children have died of exposure.
A top UN official in Nairobi says about 500,000 Kenyans need urgent help.
The latest developments came as anti-government protests fizzled out and the president said he might accept opposition demands for a fresh election, but only if ordered by a court.
The UN World Food Programme said it was struggling to get food to 100,000 hungry people forced to flee their homes in the Rift Valley area.
The International Committee of the Red Cross issued an urgent appeal for more than £7m of aid.
The BBC's Karen Allen in the Rift Valley town of Eldoret, where at least 30 people were burned to death in a church this week, says 30,000 local people have been displaced.
Some 10,000 people there are seeking refuge in the compound of the cathedral of the Catholic Church, which is now spearheading local relief efforts.
A statement by a group of independent UN rights experts on Friday said: "We are profoundly alarmed by the reports of incitement to racial hatred and the growing frictions between the different ethnic groups in Kenya."
The officially-declared results of the 27 December presidential poll - giving victory to incumbent President Mwai Kibaki over opposition rival Raila Odinga - unleashed a wave of violence.
Protesters furious at alleged electoral fraud went on the rampage, killing scores of people and torching churches, businesses and homes.
The opposition Orange Democratic Movement party has laid down four conditions to resolve the political deadlock.
Kenya has been dogged by ethnic tensions since independence
They want Mr Kibaki to stand down as president; an international body appointed to adjudicate in the dispute; and in the meantime, a transitional authority established to oversee steps to condition four: a new presidential election in three months' time.
A Kenyan government spokesman, Alfred Mutua, said the opposition's three-month deadline smacked of "blackmail".
He told the BBC: "What we are clearly saying is that Kenya is not a banana republic, Kenya is not a war-torn country.
"We have court systems and laws and orders and what we are saying is that let things follow the procedures like they do in other countries with constitutions."
South African Archbishop Desmond Tutu met Mr Kibaki on Friday and said the president was not averse to the idea of a coalition.
In other developments:
- Top US diplomat Jendayi Frazer arrived in Kenya for talks aimed at bringing the two sides together
The unrest hits Kenya's tourism as visitors cut short holidays
- UN Secretary General Ban Ki-moon urged calm in separate telephone conversations with Mr Kibaki and Mr Odinga
- French Foreign Minister Bernard Kouchner said he believed the Kenyan elections had been rigged.
A massive security force presence on Friday prevented opposition demonstrators from holding a mass rally in central Nairobi but there were clashes in the eastern port of Mombasa and the western city of Kisumu.
Kenyan politics has been dogged by ethnic tensions since independence in 1963.
Mr Kibaki depends for support on the largest ethnic group, the Kikuyus, while the western Luo and Kalenjin groups - who seek greater autonomy - back Mr Odinga.
KENYA'S ETHNIC GROUPS
Population 34.5m, comprising more than 40 ethnic groups
Kikuyu are the largest tribe, mostly concentrated around Nairobi
Most of Eastern/ North-eastern regions sparsely populated with ethnic Somalis
Main ethnic groups are:
Other African: 15%