Chadian President Idriss Deby accused a European Union military force in the east of his country of "closing its eyes" to attacks by rebels who have captured several towns in a fast-moving advance.
Mr Deby's sharp criticism of the European force (Eufor), which has a United Nations mandate to protect nearly half a million civilian refugees in Chad, followed a statement from the rebels saying they had seized the eastern town of Biltine.
It was the third town to be attacked in three days by the insurgents, whose columns of armed pick-up trucks have pushed westwards from the border with Sudan into eastern Chad, where Eufor troops are protecting a string of refugee camps.
Mr Deby has long accused eastern neighbour Sudan of backing his rebel foes. Sudan denies this, but has accused Chad of supporting Sudanese rebels who attacked Khartoum last month.
Several hundred Irish troops are taking part in an EU mission, known as Eufor, which has a UN mandate to protect civilians in Chad's eastern borderlands.
In a broadcast to the nation, the Chadian leader said his government had requested protection from the international community and had been happy to receive the EU military contingent when it deployed earlier this year.
"But we've been surprised to see that, in its first hostile test, this force has rather cooperated with the invaders, allowing humanitarian workers' vehicles to be stolen and their food and fuel stocks burned and closing its eyes before the systematic massacre of civilians and refugees," he said.
"We have the right to ask ourselves about the effectiveness of this force, of the usefulness of its presence in Chad".
A Eufor spokesman in Paris said he had no comment on Mr Deby's criticism. There has been only limited confirmation from independent sources of the scale of the rebel attacks or the identity or number of casualties.