UN Secretary General Kofi Annan has made an urgent appeal to various governments to give more money to ease the humanitarian crisis in Darfur.
Some one million people have fled their homes
Military observers from the African Union in the Sudanese region said pro-Khartoum militias had committed new atrocities, burning civilians alive.
The Janjaweed are accused of ethnic cleansing against non-Arabs, a million of whom have fled their homes.
The UN said it has received less than half of the $349m it requested for aid.
"The secretary general has been appealing to governments
worldwide to contribute or increase their financial support
to this appeal," a UN spokeswoman said.
The organisation has not named the countries, but US newspaper the Washington Post reports that they include Japan, Germany, France and Saudi Arabia.
However, many countries accused of failing to provide adequate funds say they channel money through private aid agencies or directly to governments, says the BBC's Susannah Price, at the UN.
Meanwhile, the African Union has asked for British and Dutch help in airlifting 300 African troops to Sudan.
The US said on Wednesday almost 200,000 people had fled Sudan into neighbouring Chad, and the number was still growing.
The US Deputy Secretary of State for refugees, Linda Thomas, said the situation in the six refugee camps she had visited in Chad was extreme.
She said the aid effort was struggling to improve the desperate situation and that relief workers were working "under horrendous, deplorable living conditions" that in some cases put them at risk.
Heavy rains have not helped.
The BBC's Andrew Simmons in Chad saw a convoy of aid trucks leaving to distribute supplies which got stuck overnight in the mud of a river bed.
It took several hours to free them the next day.
Meanwhile, an internal African Union (AU) report seen by the BBC says the organisation has formally asked Britain and the Netherlands to help in airlifting troops to Darfur to protect its military observers.
BBC world affairs correspondent Mark Doyle says in practice the assistance may spread to other areas, and that the Sudanese government may object to intervention from the West.
An AU fact-finding team visited a village in Darfur to investigate reports of an attack on 3 July, finding evidence of an "unwarranted and unprovoked attack".
It said that the "entire Ehda village had been burnt and deserted, except for a few men".
In Khartoum people have protested against foreign intervention
It said men, "believed to be Janjaweed", rode into a village on horseback, looted the market and chained people up before setting them on fire.
Although it did not implicate the Sudanese government directly, correspondents say criticism of pro-government forces is rare from other African countries.
The AU statement said it was actively considering turning its military observer mission into a fully fledged peacekeeping force, with particular emphasis on disarming the Janjaweed.
The UN Security Council is this week set to consider a resolution imposing sanctions.
US Secretary of State Colin Powell said on Wednesday the international community must act swiftly to help the refugees from Darfur, but he has so far said it is premature to speak of military intervention.
Sudan denies backing the Janjaweed, says that sanctions are not the answer and has warned that Sudanese soldiers would fight back if foreign troops are sent to Darfur.