The US ambassador to the United Nations has hit back at comments by the UN deputy chief about US policy on the UN.
Mr Annan (R) says he will stand by his deputy's remarks
The UN's Mark Malloch Brown said Washington did not stand up for the UN against domestic critics and used the organisation as a diplomatic tool.
John Bolton called the remarks "a very, very grave mistake" and called on Kofi Annan to repudiate his deputy's speech.
Observers say the row is linked to the possibility the US may withhold payment if the UN does not reform management.
The organisation could run out of money at the end of June and the US might withhold its dues unless there is enough progress on reforms, the BBC's Laura Trevelyan reports from the organisation in New York.
The row reflects the long-standing tensions between the UN and its biggest paymaster, our correspondent adds.
The dispute was sparked by Mr Malloch Brown's speech on Tuesday.
"Much of the public discourse that reaches the US heartland has been largely abandoned to its loudest detractors, such as Rush Limbaugh and Fox News," he said in New York.
Depending on the UN while tolerating "too much unchecked UN-bashing and stereotyping" was "simply not sustainable", he said.
"You will lose the UN one way or another."
UN officials said the speech was intended as a warning signal about the looming budget crisis - but his comments drew a strong response from Mr Bolton.
He accused the official of employing "a condescending, patronising tone about the American people".
He said he told the secretary general: "I've known you since 1989, and I'm telling you, this is the worst mistake by a senior UN official that I have seen in that entire time."
"Even though the target of the speech was the United States, the victim, I fear, will be the United Nations," he told reporters after speaking with Mr Annan.
A spokesman for the UN chief said he stood by his deputy's remarks, who said he had been appealing for engagement from Americans.
"Engage here, engage consistently and go out and engage with the American public to say the UN matters, and for the life of me, I can't understand how that can be construed as an anti-American speech," Mr Malloch Brown said.
Our correspondent says that with elections to the US Congress due in November, UN officials fear Washington is calculating that bashing the United Nations could secure some Republican votes.