Chadian officials on Monday said Sudanese military aircraft had violated Chad's air space.
Experts estimate that 200,000 people have been killed and 2.5 million more driven from their homes in four years of rape, pillage and murder in Darfur, which Washington calls genocide. Khartoum disputes the death toll and denies any genocide in its vast western region.
El-Neby and other rebel Sudan Liberation Movement (SLM) commanders are preparing a conference to form a united position ahead of a new African Union and UN mediated push for peace talks. One of the main obstacles to talks has been rebel divisions. There are more than a dozen rebel factions.
Split rebels ceasefire
In a joint statement with visiting US governor of New Mexico Bill Richardson last week, Sudan agreed to allow the rebels to hold the conference and also to a 60-day cessation of hostilities.
Some rebel commanders meeting on Tuesday in the Chadian border town of Abeche have rejected the ceasefire. But el-Neby and other SLM commanders have agreed to honour the truce.
The African Union, monitoring the violence in Darfur, could not immediately confirm the bombing on Tuesday. But the AU has complained twice previously that the government had bombed rebel areas immediately following AU visits and meetings with commanders.
Any bombing is likely to further delay any rebel conference, and the push for new peace talks.
Only one of three rebel negotiating factions signed a May peace deal with the government. Many of those who rejected the deal formed a military alliance and renewed hostilities with the government.
Mostly non-Arab rebels took up arms in early 2003 in Sudan's remote west, accusing the central government of neglect.