Time is running out for the United States to formulate a new strategy to revive negotiations with North Korea, a senior North Korean diplomat said, as a US envoy arrived in Seoul ahead of a visit by President Donald Trump.

US-North Korean dialogue has stalled since a second summit between Trump and North Korean leader Kim Jong Un collapsed in Hanoi in February after US calls for North Korea’s complete denuclearisation and Kim’s demands for sanctions relief.

Kim has said a third meeting would be possible only if Washington is more flexible and set a year-end deadline.

Washington says it is prepared to resume talks with no pre-conditions and South Korean President Moon Jae-in said on Wednesday that the United States was in behind-the-scenes talks with North Korea over a possible third summit and had proposed reopening working-level negotiations.

However on Thursday, Kwon Jong Gun, director-general for US affairs at North Korea’s foreign ministry, said that while Washington continued to speak of dialogue, it had become “more and more desperate in its hostile acts”.“If the United States is to move towards producing a result, time will not be enough,”he added.

On Wednesday, North Korea’s foreign ministry complained about a recent sanctions extension by Washington, calling it an outright challenge to the first summit between Kim and Trump in Singapore last year at which the two sides pledged to improve relations.

Trump is due in South Korea at the weekend and in the run-up to his visit, US officials raised hopes for a restart of talks with North Korea following a cordial exchange of letters between him and Kim.