North Korea's fury over anti-Pyongyang leaflets launched from the South is driven by "dirty, insulting" depictions of leader Kim Jong Un's spouse, Russia's top envoy in the reclusive country has said.

In recent weeks Pyongyang has issued a series of vitriolic condemnations over anti-North leaflets which defectors based in the South send across the militarised border -- usually attached to balloons or floated in bottles.

The campaigns have long been a point of contention between the two Koreas, but this time, Pyongyang upped the pressure, blowing up a liaison office and threatening military measures.

One of the most recent launches -- carried out on May 31 -- had included provocative imagery of the North's First Lady Ri Sol Ju, sparking "serious outrage" in Pyongyang, according to Russian ambassador to North Korea Alexander Matsegora.

Russia is a key ally of the isolated North and Matsegora is one of the longest serving ambassadors in Pyongyang.

"The leaflets bore a special kind of dirty, insulting propaganda, aimed at the leader's spouse," Matsegora told Russia's TASS news agency on Monday.

They were photoshopped "in such a low-grade way", he added, and served as "the last straw" for the North. 

Inter-Korean relations have been in deep freeze following the collapse of a summit between Kim and US President Donald Trump early last year over what the nuclear-armed North would be willing to give up in exchange for a loosening of sanctions.

Pyongyang turned its anger against Seoul rather than Washington, despite three summits between the North's leader and South Korean President Moon Jae-in, who brokered the first Trump-Kim meeting in Singapore.

The impoverished country is subject to multiple UN Security Council sanctions over its banned weapons programmes.

The Russian diplomat also dismissed speculation that Kim's younger sister was being trained as the next leader of North Korea.

Since early June, Kim Yo Jong -- a key adviser to her brother -- has been the face of Pyongyang's highly aggressive stance towards the South over the leaflets.

North Korea blew up the liaison office days after she warned it would soon be seen "completely collapsed", and later she called Moon "disgusting" and apparently "insane".

Despite her "serious political and foreign policy experience", Matsegora said Kim Yo Jong was "rather young".

"There is absolutely no reason to say that she is being trained" to take the helm, Matsegora said.

"No one dares to call themselves number two in the country," he added. 

"I think that if you asked comrade Kim Yo Jong whether she was number two, she would answer with a strong 'no'."