Bangui - Aid groups have called on the United Nations to take "immediate action" in the Central African Republic, saying the conflict-wracked nation is "teetering on the brink of catastrophe".In a letter addressed to UN Secretary-General Antonio Guterres and published on Tuesday, six humanitarian organisations expressed their "grave concern about the rapid deterioration of the security situation" in the country.We "request your office take immediate action to prevent the country collapsing into another full-blown conflict", the letter said, adding that "at least 821 civilians have been killed since the start of the year."The aid groups - ACTED, Action Contre la Faim (Action Against Hunger), the Danish Refugee Council, Premiere Urgence, the International Rescue Committee and the Norwegian Refugee Council - said the conflict had exacerbated an already tenuous humanitarian situation in the nation, where more than two million people are going hungry."These figures paint a bleak picture of a nation teetering on the brink of catastrophe," the groups said.They called on the UN to ensure that the MINUSCA peacekeeping mission "has the financial and humanitarian resources" needed to operate in the country and that it "supports Central Africans and their representatives in the implementation of political conflict resolution and peacebuilding processes."One of the world's poorest nations, CAR was pitched into a war between Muslim and Christian militias in 2013, unleashed when President Francois Bozize, a Christian, was overthrown by a coalition of Muslim-majority rebel groups called the Seleka.They in turn were ousted by a military intervention led by France, the country's former colonial ruler, which in turn sparked some of the bloodiest sectarian violence in the country's history as mainly Christian militias sought revenge.Groups on both sides are now fighting for control of natural resources, including gold and diamonds, as well as regional influence after a conflict that saw half a million people flee the country of 4.5 million.UN aid chief Stephen O'Brien warned last week that renewed clashes showed "early warning signs of genocide" and also urged immediate action to contain the crisis.In recent weeks, at least 60 people have been killed - including at least six aid workers from a local branch of the Red Cross - in fighting between armed groups in Ngaoundaye and Batangafo in the north, Kaga-Bandoro in the centre and Alindao and Gambo to the south, witnesses told AFP.Aid workers have also been targeted in the fighting, which could force them to withdraw from especially violent areas, the groups said."We are painfully aware that any long-term withdrawal would increase the vulnerability of those very people whose survival depends exclusively on our presence and proximity", the letter said. "But we cannot operate in a country where we are the target, where our staff are continuously under fire."The United Nations has 12 350 troops and police on the ground to help protect civilians and support the government of President Faustin-Archange Touadera, who was elected last year.