By Louis Charbonneau

UNITED NATIONS, Jan 16 (Reuters) - The United Nations launched an urgent appeal on Wednesday for member states to provide $42 million in humanitarian aid to help an estimated half a million people affected by violence in Kenya.

U.N. Undersecretary-General for Humanitarian Affairs and Emergency Relief Coordinator John Holmes said the appeal had been launched in coordination with some 22 U.N. agencies and nongovernmental organizations and the Kenyan government.

Holmes told reporters he expected a "reasonably generous" response from donor countries.

More than 600 people have been killed and 250,000 left homeless by the turmoil since President Mwai Kibaki was sworn in after a Dec. 27 ballot that the leader of the opposition Orange Democratic Movement, Raila Odinga, says was rigged.

The United Nations said in a statement that about half a million people have been affected by the clashes.

Some refugees have described the violence in Kenya as genocide, an accusation that the government has hurled at the opposition.

Holmes said that as bad as the violence in Kenya has become, the term genocide -- the deliberate attempt to eliminate a religious or ethnic group or nationality -- did not apply.

The situation with refugees had begun to stabilize and the number of new refugees has fallen to a trickle, he said.

"It's on a small scale at the moment," Holmes said.

"There are tens of people in small areas who are still being displaced because their villages or their homes are being attacked, but it's nothing like ... the initial wave of violence or destruction," he said.

Some people have been returning to their homes, he added.

The U.N. statement said initial assessments showed that food, shelter, water, sanitation and disease prevention are among the top priorities in Kenya.

"As the political stalemate endures, aid workers remain concerned about possible new outbursts of violence that would trigger a further deterioration of the already difficult humanitarian situation," the statement said. (Reporting by Louis Charbonneau)

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