Beatrice Godefroy, head of the Swiss branch of Doctors Without Borders in Chad, told Reuters up to 8,000 refugees had poured across the border from Darfur last week and were living rough in the desolate area around the border town of Birak.

Many had fled without their livestock and had little food. Most were sleeping with no protection from the desert sun or freezing night-time temperatures, sheltering in dried-out river beds and under trees.

"The situation is very precarious and could deteriorate further," Godefroy said by phone from the main town of Abeche in eastern Chad. "We're very concerned by what is happening."

Supplies of drinking water were sometimes half a day's walk away and many refugees, particularly the women, were deeply traumatised. Fears of violence, however, had prevented aid workers from returning to the area in recent days.

"A second wave of bombing started on Monday, taking place near the border in the area close to Birak ... We have not been able to go back because of insecurity."

Sudan's army said it had launched a "cleansing" operation in the rebel-held mountainous border region to rid it of Darfuri and Chadian insurgents, saying they were attacking civilians.

International experts estimate some 200,000 people have died and some 2.5 million have fled their homes during the five-year conflict in Darfur, and many fear the ethnic and political violence is now spilling across the border into east Chad.

NEW WAVE "EXPECTED"

At least seven wounded were brought for treatment at Birak's health centre on Wednesday after the bombings, Godefroy said. She could not confirm if the attacks continued on Thursday.

"A new wave of refugees is very likely. We are expecting new arrivals," Godefroy said. "The concern is that we are not able to deal with the existing people, so new arrivals could then make things much worse."

Foreign aid organisations working at camps in eastern Chad, which shelter some 500,000 refugees from Darfur and displaced Chadians, have warned that escalating violence in the central African country is hampering aid work and could trigger a major humanitarian crisis.

Several rebel factions, who denounce Chadian President Idriss Deby's 18-year rule as corrupt and dictatorial, attacked the capital N'Djamena nearly three weeks ago. The fighting killed 160 people and forced the evacuation of aid workers.

The first 50 members of an Irish contingent to be deployed in Chad as part of a 3,700-strong EU peacekeeping force to protect the refugees in the country's east arrived on Thursday.

Godefroy declined to comment on whether the EU force could improve security in eastern Chad, where a rise in banditry and car-jacking has forced aid workers to take extra precautions.

"We have been working here before they arrive, and I'm sure we will still be here once they have gone," she said. "It is always very volatile in eastern Chad."