Sudanese forces have clashed with troops from the European Union Force in Chad after they crossed the border into the Darfur region, Sudan says.
The French-dominated force is 3,700-strong
A Sudanese soldier and a civilian were killed during the exchange of fire, the Sudanese army says.
The EU Force (Eufor) says it was trying to recover one of its vehicles which had accidentally strayed into Sudan.
A French soldier who was in the vehicle is still missing, and France has asked for Sudan's help to find him.
The French soldier's disappearance is the first serious incident experienced by the force. Another soldier was injured - the mission's first casualty.
A Sudanese spokesman said occupants of the Eufor vehicle shot at a checkpoint five kilometres inside Sudan, and fled when Sudanese forces returned fire.
He said three jeeps and a helicopter returned to the area after 30 minutes and there was an exchange of gunfire, which resulted in the deaths of two Sudanese nationals.
The Eufor says the vehicle had mistakenly strayed three kilometres inside the unmarked border, and an attempt to recover it was met with hostile fire.
Eufor is mandated to protect refugees from the Sudanese region of Darfur and the Central African Republic, as well as internally displaced people.
The French-dominated 3,700-strong force began deploying in eastern Chad and Central African Republic last month.