Talks have been stalled since February when a second summit between Trump and North Korean leader Kim Jong Un collapsed, after they 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 and Trump is due to fly to Seoul at the weekend for a summit with South Korean President Moon Jae-in.

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

Seoul has also recently approved $8 million in food aid for the North.

But an official at the North's foreign ministry slammed the South on Thursday, saying it was trying to raise its standing by posing as a "mediator".

In a statement carried by the North's official Korean Central News Agency, he said the US and North Korea were the "direct parties", adding: "It is absolutely not an issue where the south Korean government can meddle in."

If the North had to contact Washington, it can use the "already existing communication channel", he said, stressing it will "never be done through the South Korean government".

"South Korean officials are acting as if there are various exchanges and behind-the-scenes talks between the North and south, there are none," he added.

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

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

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.