About 12,000 people have fled western Sudan into neighbouring Chad, with more expected to follow after deadly attacks on Darfur villages, the UN has said.
Sudan's government said it had launched the attacks against fighters from the Justice and Equality Movement.
But Darfur rebels denied their men had been in the targeted villages and said as many as 200 civilians were killed.
At least two million people have been displaced by the conflict in Darfur, many fleeing west to Chad.
Rebels said government forces had attacked three villages on Friday using aerial bombardment, troops and the Janjaweed Arab militia.
Large numbers of people have been displaced in the past few days and people continue to flee the violence, according to the UNHCR, the UN's refugee agency.
"Since Friday, following the bombing in west Darfur, 12,000 people have crossed from Darfur to Chad in an area called Birak," said Helene Caux, a spokeswoman for the UNHCR.
"We expect more people to arrive. We have contacted some refugees by phone and they told us that the fighting in west Darfur on Friday and Saturday was very violent so we are just expecting more people to cross," she added.
On Friday the Sudanese military confirmed it had bombed Sirba, Sileia and Abu Surouj.
Army spokesman Brig Osman Mohamed al-Aghbash said rebels had retreated to Chad "leaving behind a huge number of dead, wounded and equipment that is currently being counted".
Human Rights Watch said 150 people had died during the raids.
The group said the return to large-scale attacks on villages showed that the Sudanese government had a total disregard for the safety of civilians.
"These dead - most of them are tribal leaders or teachers or people working for the state," a tribal leader from Abu Surouj, Yehia Mohamed Ulama, told Reuters news agency.
"Are these people rebels?"
The head of a joint peacekeeping mission by the African Union and the UN called for both sides to show restraint.
The force is due to expand to 26,000 people this year, though currently just 9,000 peacekeepers are in place.
On Saturday, the government agreed to allow the force unrestricted movement, including night flights.
The flight of more refugees into Chad comes shortly after rebels there launched an attack against President Idriss Deby, throwing the country into turmoil.
Sudan and Chad both accuse each other of harbouring rebel groups.
There is growing concern over how the interlinked situations in Chad and Darfur will develop over the next few weeks, the BBC's Amber Henshaw reports from Khartoum.
The UN's special envoy to Darfur, Jan Eliasson, said Friday's attacks represented a "very disturbing new spike in violence".
"Over the last few months, the security and humanitarian situation in Darfur and the region has dramatically deteriorated, most recently through events related to Chad," he said.
At least 200,000 people are thought to have died in the fighting between Sudan's government, Arab militias and rebel groups that began some five years ago.
The government has denied links to the Janjaweed militia, accused of trying to "cleanse" the region of black Africans.