There has been an upsurge of Sudanese refugees fleeing across the border into Chad following reports of renewed violence in the Darfur region.
More than a million people have been driven from their homes
The United Nations refugee agency says up to 500 people crossed the border close to the Chadian village of Berak.
Further north, refugees describe attacks by Sudanese government planes and militia on horseback.
The UN special representative to Sudan, Jan Pronk, confirmed that the situation in a number of areas was deteriorating.
He told the BBC there was a big gap between promises made in good faith in Khartoum, and what was happening on the ground in Darfur.
"People are still afraid because of the fighting that is continuing," he said. "It is very difficult at the moment in Darfur to get the humanitarian assistance to all the (refugee) camps."
The Sudanese government denies that it is involved in the violence and has said it will work to disarm the Janjaweed.
More than 1m displaced
Up to 50,000 killed
More at risk from disease and starvation
Arab militias accused of ethnic cleansing
Sudan blames rebels for starting conflict
But the BBC's Ishbel Matheson, who has spoken to some of the newly arrived refugees in the town of Bahai in eastern Chad, says their testimonies suggest that despite intense international pressure, the campaign against Sudanese of African descent continues.
She saw about 30 women and children who were first driven from their homes nine months ago after the Janjaweed Arab militia torched their villages following aerial attacks. They hid in the mountains eating tree leaves and drawing water from the wells at night.
Eight days ago, they say, the attacks resumed using identical tactics.
More than a million people have been driven from their homes since the conflict started 18 months ago.
A UN-brokered ceasefire between Khartoum and two rebel groups in the region, where fighting has claimed 50,000 lives, is supposed to be in effect.
About 150 Rwandan troops have arrived in Sudan to help protect African Union (AU) ceasefire monitors in Darfur.
Sudanese Foreign Minister Mustafa Ismail told reporters that 150 Nigerian soldiers would join the protection force this month.
Kagame said soldiers would use force to protect civilians, if needed
The UN has said the deployment of 2,500 AU peacekeepers is being considered, but Sudan has voiced opposition to such a large force.
In a radio address, Sudanese President, Omar al-Bashir blamed unnamed enemies for the crisis in Darfur and said his government was capable of restoring security in the region.
President Paul Kagame said the Rwandan troops would use force if necessary to protect Sudanese civilians, according to the Associated Press.
"If it was established that the civilians are in danger then our forces will certainly intervene and use force to protect civilians.
"In my view it does not make sense to give security to peace observers while the local population is left to die."
Mr Pronk has said he wanted many more African observers in Darfur.
He said that progress had been made in talks with the Sudanese government about the establishment of 20km-wide safe areas around giant refugee camps in Darfur.
Under the proposal, which is expected to be finalised in the coming days, neither soldiers nor the pro-government Janjaweed militia would be able to carry weapons of any kind in these areas.
Peace talks due to take place in Abuja on 23 August between all parties in the conflict - the Sudanese government and the two rebel groups - will go ahead, Mr Pronk said, despite uncertainty about whether the rebel groups will attend.