The World Food Program (WFP) warns that fighting in Chad could leave up to 700,000 people short of food.
The U.N. agency expressed concern that violence will delay truck convoys trying to reach refugees in eastern Chad and displaced people in Sudan's Darfur region.
The agency says the trucks must get through before the end of next month, when seasonal rains are expected to make road travel impossible.
Chad's government is fighting an insurgency that President Idriss Deby says is backed by Sudan. The Sudanese government denies the charge.
The World Food Program says it remains operational in Chad despite recent clashes that led it to evacuate non-essential staff.
The agency says it is wrapping up April food distribution in eastern Chad, where it looks after some 200,000 refugees from Darfur.
The agency feeds another 500,000 displaced Sudanese in Darfur itself.