Chad says it will not accept UN military peacekeepers on its eastern border with Sudan's Darfur region.
The UN chief has proposed 11,000 troops for Chad
The deputy foreign minister said there had never been any question of Chad receiving a military force, but only of one made up of police officers.
Earlier, the UN Security Council said it supported a call for 11,000 troops to be backed up by helicopter gunships.
The force is to protect refugees in Chad and the Central African Republic displaced by the conflict in Darfur.
The crisis in Darfur has spilled over into neighbouring Chad and the Central African Republic and UN Secretary-General Ban Ki-moon said recently that he favoured the deployment of a peacekeeping force to these areas.
Concern is growing for the 200,000 refugees who sought shelter in eastern Chad.
The conflict in Darfur has followed them across the border with attacks by Janjaweed Arab militia on camels and horseback leaving hundreds dead and 110,000 people homeless.
"For Chad, it has never been a question of receiving any military force on the eastern border but rather a civil force made up of gendarmes and police officers," Deputy Foreign Minister Djidda Moussa Outman said.
The BBC's Stephanie Hancock in the capital, N'Djamena, says the announcement will be a blow for the UN Security Council, which is expected to support the resolution within the next week.
Earlier, however, the British ambassador to the UN admitted that President Idriss Deby might only favour a police force.
"The president has doubts about the military aspect, and there can be no question of a deployment of troops unless the president contents," Sir Emyr Jones Parry told the BBC's Network Africa programme.
The UN wants to send 20,000 peacekeepers to Darfur but has not yet been able to because of resistance from the Sudanese government.
But Sir Emyr said the proposal for Chad was not a tactic to get into Darfur.
"What's clear is that what happens in Darfur and eastern Chad is related. There are movements across the border," he said.
"Having an adequate security presence in each area will help stabilise the situation. But that's not to say in any sense that what we're doing in Chad is in order to get into Darfur. It is not."