Sudanese government planes have bombed the town of Haskinita in North Darfur, a rebel official has told the BBC.
Darfur's rebels are due to start peace talks next month
Rebel Justice and Equality Movement official Abdel Aziz said the aerial attacks had killed at least 25 civilians and three of his fighters.
He said the bombardment had started on Monday afternoon and then ground forces entered the rebel-controlled town.
Sudan's government was not available for comment. African Union peacekeepers could not confirm the reports.
The Jem official said most of town's residents had fled.
These reported clashes come just weeks before the start of peace talks between rebels leaders and officials from Khartoum.
The BBC's Amber Henshaw in Khartoum says that some fear the continued fighting could derail the negotiations before they have even started.
Earlier on Monday, the government said it was investigating a raid by rebels on one of its bases last month which left 41 dead.
The British foreign office minister for Africa, Mark Malloch-Brown, is due to visit Sudan's Darfur province.
He met President Omar al-Bashir on Monday in Khartoum.
He is pushing for a cease-fire and the rapid deployment of a peacekeeping mission consisting of 26,000 United Nations and African Union troops.
At least 200,000 people are believed to have died in four years of fighting in Darfur, while more than two million have fled their homes.
Sudan's government denies charges it backs Arab militias accused of atrocities against the region's black African population.