UN chief Ban Ki-moon will visit North Korea this week for a likely meeting with the nuclear-armed state's diplomatically reclusive leader, Kim Jong-Un, South Korea's Yonhap news agency reported Monday.

If the trip goes ahead, Ban would be the first UN secretary general to set foot in the North for more than 20 years, and the first international leader to meet Kim since he formally assumed power nearly four years ago.

Citing an unidentified high-level UN source, Yonhap said Ban would visit Pyongyang in his official capacity later this week, though no precise dates were given.

The UN spokesman's office in New York declined to comment on the report, while the South Korean foreign ministry and presidential Blue House both said they were unaware of the visit.

The UN source told Yonhap that Ban was almost certain to meet with Kim Jong-Un -- a meeting which, if it happens, would mark a major diplomatic opening by Pyongyang.

Since taking over the leadership following the death of his father Kim Jong-Il in 2011, Kim has yet to receive a single head of state, and has not travelled outside the country.

The young leader has received a number of high-ranking Chinese officials in Pyongyang, but the most prominent foreigner he has met in the past four years is probably the former NBA basketball star, Dennis Rodman.

A noted breach of protocol saw Kim snub the president of Mongolia who visited Pyongyang in 2013.

"There can't be such a situation where the UN secretary general visits North Korea and does not meet with the supreme leader of the UN member state," the UN source said.

- Earlier plans scuttled -

Ban had been scheduled to visit North Korea in May this year, when Pyongyang invited him to tour the joint North-South Kaesong Industrial Zone, which lies just over the inter-Korean border.

Pyongyang withdrew the invitation at the last minute after Ban criticised a recent North Korean missile test.

There are concerns that the North is now preparing another missile test, after it reportedly issued a no-sail notice to shipping off its east coast until December 7.

Two UN secretary generals have visited North Korea in the past -- Kurt Waldheim in 1979 and, in 1993, Boutros Boutros-Ghali who met with then leader Kim Il-Sung to discuss tensions over Pyongyang's nuclear ambitions.

Analysts said North Korea would use the visit to flag its willingness to engage with the international community at a time when it is under fire over its nuclear weapons programme and human rights record.

"Pyongyang will also try to argue that the United States and South Korea have been behind the recent strains in inter-Korean relations, and that the North was open to dialogue all along," said Hong Hyun-Ik, an analyst at the Sejong Institute think tank in Seoul.

"I think Kim will meet him personally in order to talk up what he sees as his achievements with North Korea's economy," Hong said.

- Growing isolation -

The North's invitation might also have been prompted by diplomatic shifts in the region that have left North Korea looking more isolated than ever, with Seoul moving closer to Pyongyang's main diplomatic and economic ally China, and improving strained relations with Tokyo.

Earlier this month, the leaders of South Korea, China and Japan held their first summit for more than three years in Seoul.

Although the focus was on trade and other economic issues, the three declared their "firm opposition" to the development of nuclear weapons on the North Korean peninsula.

North Korea is already under a raft of UN sanctions imposed after its three nuclear tests in 2006, 2009 and 2013.

It has also come under increasing pressure on the human rights front, following a report published last year by a UN commission which concluded North Korea was committing human rights violations "without parallel in the contemporary world".

It will not be Ban's first visit to the North. He crossed the border to visit the joint industrial zone of Kaesong with a delegation of foreign diplomats in 2006 when he was South Korea's foreign minister.