The US ambassador to South Korea, Mark Lippert, has been slashed on his face and arm by a lone, knife-wielding activist opposed to ongoing US-Korean military drills.

The ambassador was recovering from surgery after the attack with a paring knife slashed open his cheek and damaged nerves in his right hand, doctors said.

The diplomat was treated for two-and-a-half hours by a plastic surgeon and an orthopaedic surgeon, and will remain in hospital under observation for the next three to four days.

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“The surgery was very successful and the patient is in a stable condition and resting in his hospital room,” Chung Nam-sik, head of the Yonsei University Health System, told a press briefing.

One of the surgeons, Yoo Dae-hyun, said if the deep cut on his right cheekbone had been a few centimetres lower, it might have cut his carotid artery “which would have been life-threatening”.

The cut on the cheek was 11cm long and 3.0cm deep, but there was no nerve damage.

There was, however, some minor damage to sensory nerves in his hand which was successfully treated during the surgery, the doctors said.

Photos showed a gash on Lippert’s face, starting under his right cheekbone and extending diagonally across his cheek toward his jawbone.

Witnesses described how a man with a blade concealed in his right hand had lunged across a table at Lippert at a breakfast function on Thursday at the Sejong Cultural Institute in central Seoul.

The assailant, identified as Kim Ki-jong, 55, was immediately wrestled to the ground. Yonhap TV showed men in suits and ties piled on top of the attacker.

As he assaulted Lippert, Kim screamed a slogan in favour of reunifying the divided Korean peninsula, and later shouted his opposition to joint US-South Korea military exercises that began on Monday.

Kim was known to police, having been handed a two-year suspended sentence in 2010 for throwing a rock at the then-Japanese ambassador to Seoul.

US ambassador to South Korea Mark Lippert. Photo: Getty

District police chief Yoon Myung-soon said Kim had slashed the ambassador with a 25-centimetre paring knife.

The US State Department confirmed Lippert’s injuries were not life-threatening and said it “strongly condemned this act of violence”.

The White House said President Barack Obama had called the ambassador “to tell him that he and his wife Robyn are in his thoughts and prayers, and to wish him the very best for a speedy recovery”.

Lippert was part of Obama’s inner circle during the then senator’s rise to the White House.

He took on senior roles in national security and defence after the 2008 presidential campaign, before becoming ambassador to Seoul in October last year.

South Korean President Park Geun-hye condemned the “intolerable” assault, saying it was tantamount to an attack on the South Korea-US military alliance.

Park, who is currently on a tour of Gulf states, vowed a “thorough investigation”, while the foreign ministry said it would beef up security for foreign envoys.

A spokesman for the Korea Council for Reconciliation and Co-operation, which hosted the breakfast function, apologised for the lack of security at the event.

“This man suddenly jumped out of the audience seat when the breakfast was about to start at the table,” the spokesman said.

“Other people tried to stop him but the situation unfolded too quickly,” he added.

Kim runs a small activist group that pushes for reunification with North Korea and regularly organises protests against Japanese territorial claims to a group of small islands controlled by South Korea.

According to intelligence sources cited by the Yonhap news agency, Kim visited North Korea six times between 2006 and 2007 and tried to erect a memorial in Seoul to the late North Korean leader Kim Jong-Il after his death in late 2011.

Writing on the group’s blog on Tuesday, Kim had complained that the joint US-South Korea drills were blocking dialogue between North and South Korea and preventing reunions for family members divided by the 1950-53 Korean War.

-AAP