Bolton has been a tough critic of the United Nations

Mr Bolton, currently the top US arms control expert, is known to be a harsh critic of the UN.

His appointment was quickly criticised by opposition Democrats, who said the move sent out "all the wrong signals".

Mr Bolton must be confirmed by the US Senate before he can succeed John Danforth, who retired in January.

His nomination was announced by Secretary of State Condoleezza Rice, who described him as a "tough-minded diplomat" who "knew how to get things done".

His track record, she said, was proof of his commitment to multilateral action and reform of the UN.

But Senator John Kerry, defeated in last year's presidential election, asked: "Why would [President Bush] choose someone who has expressed such disdain for working with our allies?"

'No such thing as UN'

BBC state department correspondent Jonathan Beale says Mr Bolton's confirmation hearing in the Senate is likely to be stormy.

The president and I have asked John to do this work because he knows how to get things done

Condoleezza Rice

Profile: John Bolton

He has been a leading hawk on Iran, Iraq and North Korea, admired by many Republicans and neo-conservatives, but not seen as particularly close to former Secretary of State Colin Powell.

He has in the past been quoted as saying there is no such thing as the United Nations.

At the UN, diplomats gave the appointment a cautious welcome.

"I hope that once he is here he will have a deeper perception of what the UN is about," said Chinese Ambassador Wang Guangya.

A spokesman for UN Secretary General Kofi Annan said: "We have nothing against people who hold us accountable."

'Tough-minded diplomat'

Mr Bolton has recently been involved in multilateral talks with North Korea over its nuclear programme.

He has often accused UN bodies of not taking a tough enough stance against nations such as Iran and North Korea.

He was excluded from talks on North Korea's nuclear programme after trading insults with the country's ruler, Kim Jong-Il.

But the US administration says Mr Bolton will work well at the UN.

According to Ms Rice, he will be a "strong voice for reform", while also helping "to build a broader base of support" for the UN in the US.

Mr Bolton has been undersecretary of state for arms control and international security since May 2001.

Before that, he held a variety of senior positions in the State and Justice departments under Republican administrations.