Ongoing clashes in the Central African Republic are preventing aid workers from helping more than 42,000 people who have fled their homes in the latest flare-up.
UN Under-Secretary-General for humanitarian affairs Stephen O'Brien said on Friday (local time) he was "extremely alarmed" by the violence that erupted in the capital Bangui nearly a week ago, leaving at least 36 dead.
"Ongoing violence by armed groups is preventing humanitarian organisations from reaching more than 42,000 women, men and children who have fled for their lives in the past few days in the capital Bangui," he said in a statement from New York.
O'Brien called for an end to the violence to allow aid workers to reach those in need and warned that blocking the humanitarian effort is a violation of international law.
"People who have fled for their lives need food, water, emergency shelter and health care services," he said.
The offices of several relief organisations were looted on Wednesday and some staff were forced to relocate for their own safety, but O'Brien said the aid groups were determined to stay.
Some 2.7 million people - more than half of the population - depend on humanitarian aid to survive in the Central African Republic, which has been struggling to recover from sectarian violence triggered by a 2013 coup.
The violence has driven more than 412,000 people from their homes.
The UN aid chief said the situation was "deteriorating" outside Bangui with roaming bands of armed groups spreading panic.
The latest round of violence broke out Saturday after a motorcycle-taxi driver was murdered in the Muslim-majority PK-5 neighbourhood of Bangui.