By Stephanie Hancock
BBC News, N'Djamena
Chad says it is in "a state of war" with neighbour Sudan over the security crisis in the east of the country.
The Chad rebels want to oust Idriss Deby's government
It accuses Sudan of being the "common enemy of the nation" after a Chadian rebel attack on a town last week.
In a statement, the government calls on Chadians to mobilise themselves against Sudanese aggression.
Relations between the two states have deteriorated since Chad accused Sudan of being behind Sunday's attack on Adre, which left about 100 people dead.
The strong language in the statement will alarm observers who have already warned that tensions along the Chad-Sudan border are nearing breaking point.
In the aftermath of Sunday's attack, Chad accused Sudan of being directly involved in helping to support the Chadian rebels.
But the statement issued by Chad's government on Friday afternoon is the most aggressive yet.
It claims that not only was Sudan behind the attack on Adre, but it also accuses Sudanese militia of making daily incursions into Chad, stealing cattle, killing innocent people and burning villages on the Chadian border.
"Chad is today in a state of war with Sudan," the statement says.
It asks Chadians to form a patriotic front against what it calls "the common enemy of the nation".
The statement thanks the international community for its support so far, but says condemnations of the recent violence in Adre do not go far enough.
It appeals to the international community, including the African Union, to specifically condemn what it alleges is Sudan's involvement in the attack on Adre.