North Korea was meant to freeze its nuclear programme under a 1994 deal

The North said Christopher Hill was welcome to visit, so long as the US was prepared to "implement truthfully" a deal reached last September.

That agreement, under which the North pledged to scrap its nuclear programme, has since been cast into doubt.

A consortium set up to build reactors for the North, under a previous deal, formally aborted the plan on Wednesday.

Kedo (Korean Peninsular Energy Development Organisation), whose executive board includes directors from the US, the European Union, South Korea and Japan, said it decided to act because of Pyongyang's "continued failure" to co-operate with efforts to give up its nuclear weapons programme.

KEDO DEAL West agreed in 1994 to supply N Korea fuel oil and build it two light-water reactors Main funding came from South Korea, US and Japan N Korea was to freeze nuclear weapons programme But deal broke down in 2002 after North allegedly admitted uranium programme

The project was suspended in 2002 after the US accused North Korea of reneging on its part of the deal by allegedly running an illegal programme to enrich uranium for weapons' production last year.

South Korea expressed regret that the Kedo project had been formally killed off.

"The government thinks it is regrettable that the light-water reactor project ended without accomplishing its original goal," the South's unification ministry said in a statement.

Despite the move, Pyongyang on Thursday invited Mr Hill to visit, suggesting a continued commitment to the nuclear talks process.

"If the United States has made a political decision to truly carry out the joint declaration, [we] again invite the head US delegate in the six-party talks to visit Pyongyang and directly explain [it] to us," an unnamed spokesman for the North's Foreign Ministry said in a statement carried by the state-run Korean Central News Agency.

US sanctions

The "joint declaration" referred to the September deal, under which Pyongyang agreed to scrap its nuclear programme in return for economic aid and security guarantees.

But in November the North said it would boycott further talks after Washington cracked down on Pyongyang's alleged illicit financial activities, including money laundering.

In an apparent reference to this, North Korea's statement on Thursday warned of "hardline" measures if the US continued with its policy of "hostility" and financial pressure.

There have been reports of activity in recent weeks at a missile launch site in North Korea, a possible threat that the North was ready to break its seven year moratorium on ballistic missile tests.