Two top Chadian army generals have crossed the border into Sudan and defected, Darfur-based insurgents say.
A large group of army officers deserted last year to join rebels of the United Front for Democratic Change (FUC), seeking to oust President Idriss Deby.
Sudan has accused Chad of harbouring Darfur rebels, while Chad has said Sudan backs Chadian insurgents.
An agreement was signed in Libya earlier this month to reduce tensions between the two African nations.
The Tripoli accord commits the two countries to restore diplomatic relations and cease any attempts at cross-border conflict.
FUC leader Mahamat Nour named the two alleged defectors as General Sedi Aguid and General Ishaq al-Diar, saying they were "in one of our camps on the border".
Chad hosts some 300,000 refugees who have fled the conflict in Sudan's western region of Darfur, and has helped mediate peace talks.
Rights groups have said Chadian and Sudanese militias in Darfur have launched frequent cross-border raids.
President Deby seized power in 1990 after launching a rebellion from bases in Darfur.