The U.N. chief has told states that the organization is at risk of "imminent financial collapse," citing unpaid fees and a budget rule that forces the global body to return unspent money, according to a letter seen by Reuters on Friday.

"The crisis is deepening, threatening program delivery and risking financial collapse. And the situation will deteriorate further in the near future," United Nations Secretary-General Antonio Guterres in a letter to member states dated Jan. 28.

A U.N. spokesperson did not immediately respond to a request for comment.