UN Secretary General Ban Ki-moon on Friday called on the African Union to speedily provide promised troops to halt the worsening crisis in Central African Republic.
Ban said the resignation of transitional president Michel Djotodia and his prime minister meant speedy action was needed to restore order.
France has sent 1,600 troops to Central African Republic to support the African Union force which is meant to have up to 6,000 troops but has not yet reached 3,500.
Ban said events in Central African Republic had "gone from bad to worse."
"The 6,000 mandated ceiling has not yet been filled and I urge the African Union to expedite this full deployment of these soldiers," the UN chief said at a press conference in New York.
"We must strengthen the African-led mission and do more to prevent the spiral of violence from spreading."
France has called for the deployment of a full peacekeeping mission in Central African Republic. But diplomats say the African Union has insisted that its force, officially known as MISCA, must be given a chance to halt the strife first.
The African Union has struggled to get contributions of troops and equipment however.
Ban said he had noted the resignations announced earlier Friday of transitional president Djotodia and prime minister Nicolas Tiengaye.
"I take note of their decision. I urge all political actors in the country to work urgently to restore security and law and order," he said, highlighting how at least one million people were now displaced in the capital Bangui where daily killings are still reported.
Ban called it a "terrible crisis".
"There is no functioning government unfortunately. There is a limit for the international community and United Nations when there is no functioning government, no functioning institutions," he said.