North Korea had not responded to repeated US and South Korean entreaties to resume talks since Hanoi

South Korea on Wednesday urged North Korea to hold another summit with its leader ahead of US President Donald Trump's visit to Seoul next week, while the United States said its door remained "wide open" for talks with Pyongyang.

US Special Representative for North Korea Stephen Biegun, speaking at a Washington forum, said the United States had no pre-conditions for new talks with North Korea, which have been stalled since a failed summit between Trump and North Korean leader Kim Jong Un in February.

"The door is wide open to negotiations and ... we expect and hope that in the not too distant future we will be reengaged in this process in a substantive way," he said.

Biegun said the United States was willing to discuss all commitments made by the two leaders at a first summit in Singapore last year, which included security guarantees for North Korea. However, he stressed that progress would require "meaningful and verifiable" North Korean steps to abandon its nuclear weapons program.

Speaking at the same Atlantic Council event, Biegun's South Korean counterpart Lee Do-Hoon called for a fourth summit between Kim and South Korean President Moon Jae-in.

"I urge North Korea to respond to President Moon's outstanding invitation to hold an inter-Korea summit, if possible, before President Trump visits Korea next week," Lee said.

Trump is due to visit Seoul next week for meetings with Moon after taking part in the G20 meetings in Japan.

Trump's Hanoi summit with Kim fizzled after the two sides failed to reconcile US demands for North Korea to give up its nuclear weapons and North Korean demands for a lifting of punishing US-led international sanctions.

North Korea had not responded to repeated US and South Korean entreaties to resume talks since Hanoi, although Trump said last week he had received a "beautiful" letter from Kim. Trump said he thought something positive would happen with Pyongyang but gave no details and said he was in no rush for a deal.

Washington has given no sign of a willingness to ease up on sanctions and on Wednesday announced punitive steps against a Russian financial institution for allegedly helping North Korea evade financial sanctions by assisting a company linked to Pyongyang's primary foreign exchange bank.

Biegun, who was speaking before this announcement, stressed the positive role both Russia and China have played in efforts to persuade North Korea to give up its nuclear weapons and said he had "every expectation" that Chinese President Xi Jinping would send constructive messages on the issue during a visit to Pyongyang this week.

Biegun said both Washington and Pyongyang understood the need to be flexible in approaching further nuclear talks, but he stressed that North Korean working-level negotiators had to be empowered to discuss denuclearization - something that was not the case in the run-up to Hanoi.

"We are prepared to embrace all the full set of initiatives that our two leaders committed to, but we have to discuss all of them," he said. "We can't make progress without meaningful and verifiable steps on denuclearization. It's absolutely the core of this, it's what produced this moment to begin with."

Biegun said that while there had been no working-level talks with North Korea since Hanoi, there had been "numerous communications between our governments."

However, Biegun conceded that despite more than a year of engagement with North Korea, the two sides still had no agreed definition of "denuclearization," and added: "We do consider that a very important starting point: We will never get to our destination if we don't know where we are going."