The United States has accused North Korea of being the world's leading exporter of ballistic missile technology.

Washington's top arms control negotiator, John Bolton, made the accusation during a speech in the South Korean capital, Seoul.

Too frequently, North Korea acts as if the world will keep looking the other way

John Bolton, US undersecretary of state

He said North Korea was also at the forefront of peddling ballistic missile components and technical expertise.

Last week the White House was reported to be imposing sanctions on a North Korean company for selling missile parts to Yemen, although the sanctions are largely symbolic as there is no trade relationship between the US and Pyongyang.

The BBC's Caroline Gluck in Seoul says that while there was little new in Mr Bolton's speech, it came at a sensitive time.

North Korean officials are currently in the South for economic co-operation talks and the North has made several recent gestures aimed at ending its diplomatic isolation.

Our correspondent says South Korean officials will be worried by the timing of the speech.

Attempts to push forward reconciliation with the North have been hit by President Bush's hard-line policy towards Pyongyang.

Mr Bolton, Undersecretary of State, is in South Korea for talks with foreign ministry and defence officials about North Korea's missile programme.

He said US President George Bush was factually correct to brand the North Korean regime as part of an "axis of evil" along with Iran and Iraq.

Bolton: "We see few signs of change"

Their leaders, he said continued to starve their people, and there was an axis between the regimes, along which flowed dangerous weapons and technology.

"I must say personally that this administration has repeatedly put the North on notice that it must get out of the business of proliferation," Mr Bolton told South Korean government officials, academics and businessmen.

"Nonetheless, we see few, if any, signs of change on this front.

"Too frequently, North Korea acts as if the world will keep looking the other way."

Changes 'needed'

But Mr Bolton acknowledged there had been hopeful signs of potential change in Pyongyang, including attempts to improve its diplomatic relations with South Korea and Japan, and reports of economic changes.

Mr Bolton said the US was prepared to talk to the regime and take steps with its allies to help the country transform itself.

But he said the responsibility lay with Pyongyang.

"North Korea today faces a choice," he said. "If North Korea wants to have a brighter future, it needs to fundamentally shift the way it operates at home and abroad."

Mr Bolton also warned that a 1994 power deal under which Pyongyang agreed to freeze its suspected nuclear programme in exchange for two light water reactors could be further delayed if the North did not allow in nuclear inspectors.

Asked why he had chosen to make his comments at such a sensitive time, Mr Bolton told reporters he was simply "having a little straight talk among friends".