"The reality is the contrary," senior foreign ministry official Kwon Jong Gun said in a statement carried by the state news agency KCNA.

"The south Korean authorities would better mind their own internal business," he added, in a stinging rebuke to the North's neighbour days before US President Donald Trump arrives in Seoul amid a nuclear deadlock.

Negotiations between Pyongyang and Washington have been stalled since February when a second summit between Trump and North Korean leader Kim Jong Un collapsed in Hanoi as the pair failed to agree on what the North would be willing to give up in exchange for sanctions relief.

The two sides have had minimal contact since -- aside from exchanges of letters between their leaders.

The dovish Moon said Wednesday that "behind-the-scenes talks" were going on between the US and the North with a view to preparing for a third summit.

"Also under way is dialogue between the South and the North through diverse channels," he added in a written interview with leading media.

But Kwon, the director-general of the North's Department of American Affairs, slammed the South on Thursday, denying that "various forms of exchanges and closed-door meetings" were under way between the Koreas.

Southern authorities were trying to "make their presence felt" by posing as a mediator, he said.

Dialogue between Pyongyang and Washington "would not open by itself", he went on, but if the North had to contact Washington it would use "the liaison channel already under operation".

"The south Korean authorities have nothing to meddle in," he added.

Moon, who favours engagement with Pyongyang, brokered last year's talks between Trump and Kim, and has held three summits with the North Korean leader.

There was even speculation last autumn that Kim could visit Seoul, but Pyongyang has repeatedly criticised the South since Hanoi, even though it recently approved $8 million in food aid for its neighbour.

- 'Like a parrot' -

Trump is due to fly to Seoul at the weekend for a summit with South Korean President Moon Jae-in.

The North's statement Thursday also reminded Washington that it did not have much time left until Kim's end-of-year deadline to adopt "a correct method of calculation" to revive dialogue.

"The US 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.

Trump and Kim held a groundbreaking summit in Singapore last year -- the first-ever meeting between a North Korean leader and a sitting US president -- where the pair signed a vaguely worded deal on denuclearisation.

But in Hanoi, Washington accused Pyongyang of effectively demanding an end to all sanctions for partial disarmament, while the North said it wanted some measures eased in return for closing all the nuclear facilities at its Yongbyon complex.

While the North has not responded to US offers of working-level talks, the two leaders have maintained written contact.

On Sunday, the North's state media said Kim received a personal message "of excellent content" from the US president, just days after Trump said Kim had sent him a "beautiful letter".

It was the 12th letter exchanged between Kim and Trump since the start of 2018, according to Kim Yeon-chul, the South's Unification Minister in charge of inter-Korean affairs.

Kim has written eight of them and Trump four, he told reporters.

"I guess those leaders recognise the importance of resuming the dialogue between the two countries as they are constantly exchanging letters."

Trump's visit to the South had fuelled media speculation over a possible trilateral summit with Kim and Moon, but Trump himself has said he will not be meeting Kim.