Fighting in Sudan's Darfur region has spilled over into Chad, where the army has clashed with pro-Sudanese government militias, Chad says.
The 'Janjaweed' militia are accused of ethnic cleansing
The Arab "Janjaweed" militia have been accused of ethnic cleansing against the black population of the Darfur region.
Some 1m people have fled their Darfur homes since a rebellion a year ago.
World Food Programme head James Morris said Darfur was "one of the world's worst humanitarian crises" but the US has ruled out an international force.
An estimated 10,000 people have been killed.
"Our forces clashed with the Janjaweed. We lost a commandant... The Chadian government strongly condemns this clash," said acting Chad Defence Minister Emmanuel Nadingar.
He also said that six civilians were killed, reports Reuters news agency.
The Sudan government denies backing the Janjaweed but refugees say that the militias, mounted on horses and camels, often follow up air-raids by government planes by attacking villages, killing, raping and looting.
Some one million people have fled the fighting
Sporadic clashes have continued despite a ceasefire signed by the government and two rebel groups in Chad last month.
But US Secretary of State Colin Powell on Wednesday ruled out sending foreign troops to stop the fighting.
"There is no army that is going to go in there and put down the insurrection," he said.
"We have got to use the pressure of the international community on Khartoum."
Mr Morris told the BBC that food was available for only about half of the displaced people.
He said that without further assistance the people would "be put severely at risk" as the rainy season approaches.
"Everything has been taken away from these people. This is tragic," he said, adding that he had never before seen such frightened people.
Mr Morris added that security was the key issue and for this the Sudanese government was responsible.
He said it was up to the government to bring the Janjaweed under control.
The UN has accused Sudan of backing the militias in a campaign of "ethnic cleansing" against black residents.
Two rebel groups in Darfur - the Sudanese Liberation Movement (SLM) and the Justice and Equality Movement (Jem) - took up arms last year, accusing the government of ignoring the region.