The United Nations says fighting between Chadian troops and rebels in eastern Chad has forced civilians to flee into Sudan's already-troubled Darfur region.

The U.N. High Commissioner for Refugees said Tuesday that at least 8,000 people, including an "undetermined number of Chadians," have set up camp on the Darfur side of the border.

Officials said the number of Chadians is unclear, because the refugees also include Sudanese nationals who had crossed into Chad, fleeing the ongoing conflict between their government and rebels. 

The fighting in eastern Chad has been centered around the town of Adre, which rebels attacked in December.

Since that attack, Chad has withdrawn its troops from border garrisons to protect main frontier towns, creating a dangerous security vacuum along the Sudan-Chad border.

Sudan has accused Chad of harboring Darfur rebels, while Chad has said Sudan backs Chadian insurgents seeking to oust its president, Idriss Deby.

Earlier this month, both sides signed a Libya-brokered accord to stop supporting rebels on each other's territory.