The decision to appoint John Bolton on Monday upset Democrats in Congress, who had hoped for a less contentious choice as the US representative at a time of tense UN-Washington relations.
But Secretary of State Condoleezza Rice said: "The president and I have asked John to do this work because he knows how to get things done. He is a tough-minded diplomat."
Bolton, 56, who has been undersecretary of state for arms control and international security since May 2001, has been openly critical of Iran and North Korea.
Critical and criticised
Bolton's nomination must be confirmed by the US Senate, where Democrats were expected to bring up dismissive comments he has aimed at UN effectiveness and a disdain for some international treaties.
"The [UN] Secretariat building in New York has 38 stories. If it lost 10 stories, it wouldn't make a bit of difference," Bolton said in a 1994 panel discussion sponsored by the World Federalist Association.
Senator Barbara Boxer, a California Democrat and member of the Senate Foreign Relations Committee, cited this quote at a confirmation hearing for Bolton for his current post as evidence that he was outside the US mainstream.
And Senator John Corzine, a New Jersey Democrat, said Bolton was the wrong choice when the US was seeking to repair diplomatic ties frayed in Bush's first term.
"Now, when bridge-building and strengthening of alliances are so critical to our national security, he is a poor person to serve as a conciliator at the United Nations," he said.
If approved, Bolton will succeed former US senator John Danforth, who resigned in December.
Bolton, who keeps a model hand grenade conspicuously on a table in his office, stressed what he called "his support for effective multilateral diplomacy" when he appeared with Rice for the nomination announcement.
"Close cooperation and the time-honoured tradition of frank communication is central to achieving our mutually-held objectives. The United Nations affords us the opportunity to move our policies forward together with unity of purpose," he