Democrats in the U.S. Senate say they will challenge the nomination of John Bolton to be the next U.S. ambassador to the United Nations.

Several senators have expressed concern over statements Mr. Bolton has made criticizing the world body's bureaucracy and some of its peacekeeping missions.

Democrat Christopher Dodd of Connecticut said Mr. Bolton's hostility to the United Nations will keep him from being an effective ambassador.

Republican Richard Lugar, chairman of the Senate Foreign Relations Committee, told reporters he was reserving comment on the nomination.

Mr. Bolton is known for a blunt speaking style that has angered other countries. In his role as undersecretary of state for arms control, he once described North Korean leader Kim Jong-il as a tyrant who has turned his country into a "hellish nightmare."

Pyongyang responded by calling Mr. Bolton "human scum."