(Adds U.S. comment)
SEOUL, June 27 (Reuters) - 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 U.S. envoy was due to hold talks in Seoul ahead of a visit by President Donald Trump.
U.S.-North Korean dialogue has stalled since a second summit between Trump and North Korean leader Kim Jong Un collapsed in Hanoi in February after U.S. 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 U.S. 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".
"The dialogue would not open by itself, though the United States repeatedly talks about resumption of dialogue like a parrot without considering any realistic proposal that would fully conform with the interests of both sides," Kwon said in a statement carried by Pyongyang's official KCNA news agency.
"If the United States is to move towards producing a result, time will not be enough."
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 after visiting Japan for the G20 summit and in the run-up to his visit, U.S. officials raised hopes for a restart of talks with North Korea following a cordial exchange of letters between him and Kim.
Trump said on Wednesday he would not meet Kim during his visit, but "may be speaking with him in a different form." He did not elaborate, but a White House official said he may have been talking about exchanging more letters with Kim.
South Korean Prime Minister Lee Nak-yon said on Thursday Trump was expected to send "some kind of message" to North Korea from the demilitarized zone (DMZ) separating the two Koreas.
The United States and North Korea held working-level meetings in the DMZ before Hanoi and Trump's visit to South Korea has been preceded by one by Stephen Biegun, the U.S. special envoy for North Korea.
On Friday, Biegun will meet his South Korean counterpart Lee Do-hoon to explore ways to reopen nuclear talks, but there has been no word on whether he will meet any North Koreans.
North Korea has shown some reluctance in the past to deal with Biegun, and Kwon said any new talks should be led by a "counterpart who has a good sense of communication".
He also dismissed the idea of Seoul mediating between Washington and Pyongyang, saying that North Korea-U.S. relations were "moving forward on the basis of the personal relations" between Kim and Trump.
U.S. "READY TO ENGAGE"
Asked about Kwon's statement, a spokeswoman for the U.S. State Department said the United States continued to invite its counterparts for talks.
The United States remains ready to engage in constructive discussions with North Korea to make progress simultaneously and in parallel towards the goal the two leaders set out at the Singapore summit of transformed U.S.-North Korea relations, building lasting peace, and complete denuclearization," she said.
Trump tried to go to the DMZ, where Kim and Moon held a historic first summit last year, in 2017, but heavy fog prevented it. U.S. officials have declined to say whether he will go this time.
On Sunday, KCNA raised hopes for a resumption of talks when it quoted Kim as saying that Trump's latest letter was of "excellent content" and he would "seriously contemplate" it.
However, neither side has given details of the exchange and there has been little sign that they are any closer to narrowing their differences.
South Korea's presidential Blue House said on Thursday Chinese President Xi Jinping, who met Kim in Pyongyang last week, told Moon in a meeting on Thursday that the North Korean leader's commitment to denuclearisation remained unchanged and he wanted to resolve the issue through dialogue.
At a monthly news briefing in Beijing, a spokesman for China's Defence Ministry suggested - as China has done previously - that sanctions could be eased on North Korea if it followed U.N. resolutions.
"The U.N. Security Council's actions should support the current diplomatic efforts to promote the denuclearisation of, and a political resolution for, the peninsula," Ren Guoqiang said. (Reporting by Hyonhee Shin; Additional reporting by Ben Blanchard in BEIJING and David Brunnstrom in WASHINGTON; Editing by Nick Macfie and James Dalgleish)