Rebels in Chad have said they are at war with the French-led European Union peacekeeping force which is due to be deployed in the coming weeks.
These rebels were captured after Monday's battle
The rebels accused French military planes of flying over their positions and passing intelligence to the government during this week's fighting.
France, the former colonial power, retains a military base in Chad.
The EU force is to be sent to the area near the border with Sudan's Darfur, to protect refugees and aid workers.
Chad says that Sudan is behind this week's attacks, because it did not want any western forces on its border.
France and Austria have both said the rebel declaration will not stop them from sending troops.
'Ready to go'
"The UFDD considers itself to be in a state of belligerence against the French army or any other foreign forces on national territory," said Mahamat Hassane Boulmaye, spokesman for the Union of Forces for Democracy and Development.
He accused France of giving "diplomatic, strategic and logistical support" to Chad's President Idriss Deby.
"This is an act of hostility and will be treated as such."
French President Nicolas Sarkozy says he will not be deterred.
"If we decided to send a European force... it is precisely because there are problems, difficulties," he said.
Austrian defence ministry spokesman Stefan Hirsch told the AFP news agency that the threat "does not change our determination to take part in this humanitarian mission."
"We are ready to go," he said.
France is expected to contribute the bulk of the 3,500 troops.
The BBC's Stephanie Hancock in the capital, Ndjamena says the French have more than 1,000 troops stationed in Chad and their role has always been very controversial.
But she says the declaration of war on all foreign troops, including the French, will be treated with alarm in Chad, as France is to supply the bulk of the EU troops, which are due to start arriving in January next year.
On Thursday, a different rebel group, the Assembly of Forces for Change (RFC), warned the EU they would be seen as a "foreign occupation force" if they sided with the government of President Idriss Deby.
Both the rebels and the government say that hundreds of fighters have been killed in this week's clashes, which have shattered a month-long ceasefire.
Chad's government said it had wiped out the UFDD, which it said had been armed by neighbouring Sudan.
Chad Prime Minister Nouradine Delwa Kassire Coumakoye again accused Sudan of backing the rebels.
"[Sudanese President] Omar Hassan al-Bashir is losing sleep over the arrival of the UN and EU forces," Mr Coumakoye told reporters.
"He wants to stop the force from coming, because he thinks this force, which will be on the Chadian border, constitutes a danger to him."
Sudan insists that a separate UN peacekeeping force to be sent to Darfur, should not include any western troops. Sudan has previously denied having links to Chad's rebels.
Fierce fighting broke out on Monday between the town of Abeche and the Sudan border.
Both the RFC and UFDD were among the four rebel groups who signed up to a Libya-brokered peace deal just last month.
The proposed EU force has been hit by delays and fears that its French contingent may not be seen as neutral, given France's support for Chad's president.