Former colonial power acts to boost peacekeeping force a day after UN warns of possible civil war amid claims of atrocities

France has said it will send an extra 1,000 troops to the Central African Republic (CAR) as pressure grows for outside intervention to arrest a chaotic sectarian conflict between Christians and Muslims.

Jean-Yves Le Drian, the French defence minister, made the announcement a day after the UN warned of possible civil war in the CAR, where lawlessness prevails and armed groups have been accused of atrocities including killings, burning villages and the conscription of child soldiers.

Le Drian told Europe 1 radio: "It's in collapse and we cannot have a country fall apart like that. There is the violence, massacres and humanitarian chaos that follow a collapse. It will be a short mission to allow calm and stability to return."

There is uncertainty over whether a peacekeeping mission should be French, African or UN-led, how many boots will be put on the ground and how quickly it can be mobilised.

There are 2,500 African regional peacekeeping troops in CAR, due to be increased to 3,600 by January 2014. But they are stretched thin and struggling to maintain order in a land mass bigger than France where villages are often inaccessible by road.

France, the former colonial power, has 420 soldiers on the ground, mostly to protect the airport in the capital, Bangui, although some could be seen last week patrolling rural areas in armoured vehicles. The CAR's beleaguered central government has appealed for a stronger French presence.

On Tuesday, Le Drian announced that France would send another 1,000 troops to be deployed for "a short period, in the range of six months", as was the case with France's intervention in Mali earlier this year.

But he brushed off comparisons with that crisis, adding: "In Mali there was an attack of jihadists, terrorists who wanted to transform Mali into a terrorist state. This is a collapse of a country with a potential for religious clashes. France has international responsibilities."

Earlier, at the UN, the French ambassador, Gerard Araud, said that until the African Union force of 3,600 troops was fully operational French troops would start trying to restore law and order.

"Basically our force will in the first instance be a bridging force, before the African force is fully operational, and after that … we will support the African force," he told reporters. "Today, nothing is possible in Central African Republic without restoring law and order. So that is an absolutely priority … that's why we are going to help the African force do the job."

In a briefing on Monday to the security council, the UN deputy secretary-general, Jan Eliasson, said the situation in CAR was deteriorating so fast that a UN peacekeeping force may soon be the only option.

"A country in the heart of Africa is descending into complete chaos before our eyes," Eliasson said. "The situation requires prompt and decisive action." He warned the country was becoming a breeding ground for extremists and armed groups and unprecedented sectarian violence between Muslims and Christians.

"If this situation is left to fester, it may develop into a religious and ethnic conflict with long-standing consequences, even a civil war that could spread into neighbouring countries."

Eliasson said virtually the entire population of 4.6 million people was "enduring suffering beyond imagination" and a third of its people were in dire need of food, protection, health care, water, sanitation and shelter.

With a long history of chaos and coups, the mineral-rich CAR has been in turmoil since a coalition of rebel groups, including hundreds of mercenaries from Chad and Sudan, joined forces in March to overthrow President François Bozize. The violence has increasingly pitted the mainly Muslim fighters of the Seleka rebels against Christian militias. Christians make up half the population and Muslims 15%.

Last week, the United Nations secretary general, Ban Ki-moon, presented the security council with options for supporting the African Union-led operation financially and logistically or transforming it into a UN peacekeeping operation. Ban said he would back a UN force with nearly 11,000 soldiers and police if the crisis degenerates.

Mesmin Dembassa Worogagoi, the CAR's charge d'affaires, said "people aspire to one thing alone – peace and security", and UN support was essential to achieve this. The transition leading to elections "has been threatened by the great instability prevailing throughout our country," he added.

Amnesty International warned on Monday that the crisis was spinning out of control.

Its secretary general, Salil Shetty, said: "The international community must take action before it is too late to ensure that the abuses come to an end and that CAR isn't catapulted into the international spotlight because it became a human catastrophe."