Chad's President Idriss Deby said its border with Central African Republic had been closed pending an end to intercommunal violence there that has killed thousands and forced nearly one million people to flee their homes.
Thousands of French and African troops have failed to stop the waves of killings that erupted after the predominantly Muslim Seleka rebel movement seized power in the majority-Christian former French colony in March 2013.
Seleka leaders were forced to resign under international pressure but Christian militias known as "anti-balaka" have intensified revenge attacks against Muslims.
"From today our border with this country is tightly sealed," Deby said during a visit on Sunday to the town of Daha, 20 km (13 miles) from the border. His speech was posted on the website of the presidency on Monday.
He said the 1,000-km frontier would remain open to Chadian citizens wishing to return home.
"Barring that precise case, no one is authorized to cross this border until the crisis in Central African Republic is resolved," he said.
Chad, previously at the heart of African efforts to stabilize the neighboring country, withdrew its forces from Central African Republic last month.
Chadian troops were accused by many in Central African Republic of siding with the Seleka rebels.