The UN believes at least 10,000 troops will be required in any force sent to end unrest in Central African Republic, the French UN envoy says.Ambassador Gerard Araud described the situation in CAR as "very, very dire".
His comment comes after the UN Security Council approved a resolution allowing European troops to use force in CAR.
About a million people - 20% of the population - have fled their homes during months of religious violence, after rebels seized power last March.
Speaking to reporters, Mr Araud said the African Union force in the country, intending to reach 6,000 troops, "is considered now too low because frankly the situation is very, very dire and the country is huge".
Threat of sanctions.
The UN Security Council resolution, which was passed unanimously, allows reinforcements to use "all necessary measures" to protect civilians in the country, which has been in near anarchy since its president was overthrown 10 months ago.
In addition to the use of force, the resolution allows for sanctions against the ringleaders of groups blamed for massacres and human rights abuses.
Security Council members have been alarmed by the vicious cycle of vengeance between Muslim and Christian militias in the Central African Republic.
There is concern that without a stronger international response - the situation will degenerate into a countrywide religious divide and spiral out of control, she adds.
The EU has agreed to send up to 600 troops to help African and French troops already deployed in the country to prevent further bloodshed.
France, the former colonial power, has 1,600 troops in CAR, working with some 4,000 from African countries.
On Monday, UN human rights chief Navi Pillay said the situation was getting even worse despite the inauguration of a new leader last week.
She called for more international help, saying Muslim civilians were being targeted.
Many Christian communities set up vigilante groups, accusing the mainly Muslim rebels of attacking them.
Also on Monday, Christian and Muslims leaders asked UK Prime Minister David Cameron for more assistance.
CAR is rich in gold and diamonds but years of unrest and poor governance have left most of its 4.6 million people in poverty.