North Korea has finally responded to persistent rumours that leader Kim Jong-Un had plastic surgery to look like his revered grandfather, issuing an angry denial criticising "sordid" media reports.

Speculation that Kim's resemblance to the country's founding leader, Kim Il-Sung, is not 100 percent natural has been around for some time, though with little or no supporting evidence.

The rumours have been partly fed by the undeniable fact that Kim Jong-Un has sought to evoke memories of his grandfather in numerous ways, through his dress, haircut, gestures and public appearances.

And there is a clear physical resemblance. But Pyongyang's patience with reports of plastic surgery appears to have run out.

"The false report... released by enemies is a hideous criminal act which the party, state, army and people can never tolerate," the official Korean Central News Agency (KCNA) said in a commentary Wednesday.

Insisting that the very idea of Kim, who is in his late 20s, undergoing plastic surgery was "unimaginable," KCNA flatly rejected what it described as "sordid hackwork by rubbish media."

While the focus of the commentary's fury was the South Korean media, it seems Pyongyang's anger was triggered by a Chinese report last week that was widely picked up by newspapers in the South.

China's Shenzhen TV cited a diplomatic source who, on a private visit to Pyongyang, had spoken to a North Korean official who confirmed the plastic surgery rumours.

KCNA referenced the Chinese report, but saved its real vitriol for the South Korean newspapers.

"Those hurting the dignity of the supreme leadership of the nation should not expect any mercy or leniency.

"Time will clearly show what dear price the human scum and media in the service of traitors of South Korea, slaves of capital, will have to pay," it said.

Kim Il-Sung, who died in 1994, still inspires quasi-religious devotion among many North Koreans.

He is officially referred to as "The Great Leader" and is designated in the constitution as the country's "Eternal President". His birthday is a public holiday.

Possibly concerned that the Shenzhen TV report may have caused offence, China's official Xinhua news agency issued a report Tuesday, citing two of its correspondents in Pyongyang who rejected the plastic surgery rumours.

China was embarrassed in November when the Communist Party newspaper, the People's Daily, treated as genuine a spoof article by the satirical US website The Onion anointing Kim Jong-Un as "Sexiest Man Alive" for 2012.