The top U.N. refugee official has warned of impending calamity in Sudan's Darfur region, and appealed for the immediate dispatch of a large peacekeeping force.

U.N. High Commissioner for Refugees Antonio Guterres told the Security Council Tuesday that violence and impunity in Darfur have again become everyday occurrences. He warned that the instability is spreading across the border to neighboring Chad. "I do believe the Sudan-Chad situation is the most challenging humanitarian problem we face today in the world," he said.

Commissioner Guterres appealed to the international community to urgently pressure warring factions in Darfur to end their fighting, and to send a blue-helmeted U.N. peacekeeping force to augment the seven-thousand strong African Union force on the ground.

"This is crucial, and the proof that this is crucial is that the Instability in Darfur is very quickly having an impact on Chad, and the recent events on the border are extremely worrying for us. We had to relocate part of our staff, we have more than 200-thousand refugees in Chadian border and a situation of military confrontation would be total humanitarian disasters, and my appeal is for everyone to understand these are related," he said.

Darfur's civil war began nearly three years ago, when rebels took up arms against the Khartoum government, charging state discrimination. The government replied by arming Arab militias known as Janjaweed, who unleashed a campaign of ethnic cleansing that the United States called "genocide".

Tens of thousands of people were killed, and an estimated two million more fled to refugee camps, some in Sudan, others in Chad.

The international community responded to the crisis by authorizing the African Union peacekeeping force to Darfur. But the seven-thousand A.U. troops have been unable to stop either the violence or the humanitarian catastrophe gripping the region.

Earlier this month, U.N. special envoy in Sudan Jan Pronk warned the Security Council that the current strategy for bringing peace to Darfur is in shambles. He called for formation of a new 20-thousand strong U.N. peacekeeping force to provide security in the vast region, which is the size of France.