Sudan's army and rebels have accused each other of staging attacks in the Darfur region, days after peacekeepers said security had improved.
The rebels accuse the government of discriminating against black Africans
Rebels accused government forces of using helicopters and planes to bomb villages while the army said the rebels had killed four soldiers.
A helicopter crashed in Darfur during bad weather with the deaths of 19 military personnel, a statement said.
More than 2m people have fled their homes in two years of conflict.
Four villages have been bombed since Friday, one of which was completely destroyed, Sudan Liberation Movement spokesman Mahjoub Hussein told the AFP news agency.
"Our movement is in a state of high alert and we will blame the government for any instability in the area," he said.
Armed forces spokesman Gen Abbas Abdel Rahman Khalifa said the army had not attacked villages, only rebel camps, after armed guards with a civilian convoy were attacked on the road between Darfur's two biggest towns, al-Fashir and Nyala.
African Union peacekeepers confirmed the attack on the convoy and said they were investigating the incidents.
Later, a military statement said the M-17 helicopter had crashed in the Damayat region west of Nyala after several failed attempts to land.
It said the crew had been on an "administrative" mission.
Peace talks between the two sides are due to resume next month.
US Secretary of State Condoleezza Rice has urged Khartoum to end the violence.
Ms Rice, who visited Sudan last week, said the government had a "credibility problem" regarding Darfur and demanded "action not words" to stop the violence.
The US has described the violence in Darfur as genocide.
Sudan denies backing the Janjaweed militias, accused of widespread atrocities.
Last week, the commander of the African Union peacekeeping force in Darfur, Festus Okonkwo, told the BBC that there had been no major attacks in the region since January and that there had also been a reduction in attacks on villages.
But US aid official Andrew Natsios said this was chiefly because there were no villages left to burn down.