The South's President Moon Jae-in's special envoy Chung Eui-yong, who is leading the five-member delegation, earlier said he would discuss ways to "completely denuclearise" the Korean peninsula and establish "lasting peace".

His delegation "met with Chairman Kim Jong Un and delivered a personal letter (from Moon) and exchanged opinions", a presidential office spokesman in Seoul said.

The delegation flew back to Seoul after attending a dinner banquet but Chung and other officials declined to speak to the media.

Details about their trip will be given during a press briefing on Thursday, Moon's office said.

US President Donald Trump and Kim reached a vague agreement at a landmark summit in June to work towards the denuclearisation of the Korean peninsula, but there has been little movement since.

Talks reached an impasse last month when Trump abruptly cancelled Secretary of State Mike Pompeo's trip to North Korea, citing a lack of progress.

The stated aim of the South Korean delegation's day-long visit to Pyongyang is to finalise details of a third summit between the leaders of the two Koreas, due later this month.

But observers said that Moon's personal letter to Kim will likely be a proposal aimed at breaking the denuclearisation impasse.

The envoy was likely to suggest "that Kim gives a firm commitment to presenting a list of nuclear weapons and fissile materials demanded by the US in return for a declaration of the end of the Korean War," Yang Moo-jin of the University of North Korean Studies told AFP.

Despite the deadlock with the North, Trump expressed his hopes for the success of the next inter-Korean summit in a phone conversation with Moon on Tuesday.

Pyongyang has slammed Washington's "gangster-like" demands for complete, verifiable and irreversible disarmament, and accused it of failing to reciprocate the North's "goodwill measures", including the handover of the remains of US troops killed in the 1950-53 Korean War.

When Kim and Moon met in April for their first summit, they agreed to push for a declaration from Washington of an end to the Korean War, to replace the 1953 armistice.

But US officials say the North must be rid of its nuclear weapons before that can happen.

The International Atomic Energy Agency reported last month that there is no indication North Korea has stopped its nuclear activities.