An Oxfam aid worker says he fears the peace force for Chad and the Central African Republic may contain too many French troops to be seen as neutral.
Hundreds of thousands have been displaced in Chad and CAR
The United Nations Security Council approved the force to protect civilians bordering Sudan's Darfur region.
The peacekeepers, to be made up of 3,000 European Union troops and 300 UN police, will have the right to use force to end cross-border incursions.
France is the former colonial power in both Chad and CAR.
A Chadian rebel group has already warned that the peacekeepers should not act as an intervention force.
Timane Erdimi, leader of the Rally for Forces for Change, told the BBC his men would resist the troops if they did.
The UN says there are 240,000 refugees from Sudan's Darfur region in eastern Chad and 173,000 internally displaced people.
North-eastern CAR hosts some 2,660 refugees from Darfur and some 200,000 people have been displaced by an insurgency in the north.
Under the plan which has been drawn up by France, the troops will be tasked with monitoring camps for people displaced by the violence.
'Peace has no price'
The UN refugee agency head Antonio Guterres welcomed the adoption of the resolution and called for an early deployment of the force.
The move was also welcomed by CAR's Foreign Minister Come Zoumara.
"For the Central African Republic that solution represents peace, tranquillity for our population. Children will be able to go to school; mothers will be able to go to health centres; cattle owners will be able to travel around safely; peace has no price," he said.
But Oxfam's programme director in Chad Roland Van Hauwermeiren said he had concerns.
"Will the force have enough troops? Do they have a clear mandate which is not clear at all," he told the BBC's Network Africa programme.
"And the neutrality - if the major component of the forces is French it will not be seen as a neutral force in the country by a lot of other stakeholders."
Sudan's government has agreed to a joint African Union and UN force made up of 26,000 peacekeepers on the condition that it would be predominantly African.
It is due to be deployed to Darfur by early next year in an attempt to bring an end to the four-year conflict in which at least 200,000 people are estimated to have died and more than 2m have fled their homes.