North Korea hit out Sunday at accusations that it may be behind a shocking knife attack on the US envoy to the South, branding the claims a "vicious" smear campaign by Seoul.

Kim Ki-Jong slashed Mark Lippert with a paring knife Thursday in an assault that left the US envoy needing 80 stitches to a deep gash on his face.

Kim, 55, was immediately arrested and charged with attempted murder, and police are investigating whether he has any links to the communist North.

He has reportedly told police that he had acted alone and denied any links to the North, calling the suggestion "outrageous".

The profile painted of him -- based on past brushes with the law and his blog postings -- is that of a lone assailant with strong nationalist views who saw the US as one of the main obstacles to the reunification of the divided Korean peninsula.

But Kim has also visited the North seven times since 1999, and once tried to erect a memorial in Seoul to the late North Korean leader Kim Jong-Il after his death in 2011.

Kim told police he had stabbed Lippert in protest at massive US-South joint army exercises currently underway. The annual exercises are routinely slammed by the North as a practice for invasion.

After the attack Thursday, the North hailed Kim's act as "just punishment" and a valid "expression of resistance" to the US-South military drills.

But on Sunday the North's Committee for the Peaceful Reunification of Korea (CPRK) bristled at suggestions that it might have been behind the assault, calling it an attempt to defame its leadership.

"Even the police and conservative media of South Korea joined the (South's) regime in attempting to link the case with the (North)," it said in a statement carried in English by the state-run KCNA early Sunday.

"Such moves are prompted by a vicious intention to save itself from the present awkward position... and intensify an anti-DPRK smear campaign worldwide," the CPRK said, using the North's official title.

Questions over 'mastermind'

Last week police raided Kim's house in search for evidence of his potential links to Pyongyang and found several books published in the North, a senior Seoul detective on the case said Sunday.

"We are investigating if there were any mastermind behind the attack," Kim Doo-Yeon told reporters, adding police were probing whether Kim had violated the anti-communist national security laws.

The notorious laws ban unauthorised contact with the North or activities deemed to be praising Pyongyang, which technically remain at war with Seoul after the 1950-53 Korean War ended with a ceasefire instead of a peace treaty.

Offenders are subject to up to seven years of imprisonment.

The laws -- often used to stifle political dissents under the South's army-ruled governments in the past -- have been criticised by rights groups for curbing freedom of expression.

Kim, who also voiced anger at the South's former colonial ruler Japan on his blog postings, was convicted in 2011 of hurling a stone at then-Japanese ambassador and received a suspended jail term.

Lippert, 42, is recovering well and expected to be released from the hospital as early as Tuesday, his doctors say.