Seoul - The US ambassador to South Korea, Mark Lippert, was recovering from surgery Thursday after being slashed on his face and arm in Seoul on Thursday by a blade-wielding activist opposed to ongoing US-Korean military drills.

The United States condemned the "act of violence" which left the ambassador bleeding profusely and saw him rushed to hospital where his condition was described as stable after two-and-a-half hours of treatment by plastic and orthopaedic surgeons.

Witnesses described how a man armed with a 25-centimetre (10-inch) paring knife had lunged across a table and attacked Lippert at a breakfast function in central Seoul.

The assailant, dressed in traditional Korean clothes and identified as Kim Ki-Jong, 55, was immediately wrestled to the ground and taken into police custody.

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

He was a known political activist who had been handed a two-year suspended sentence in 2010 for throwing a rock at the then Japanese ambassador to Seoul.

- Nerve damage -

One of the doctors who operated on the ambassador said if the deep cut on his cheek had been just a little lower it might have severed his carotid artery "which would have been life-threatening."

The hospital said there was some damage to sensory nerves in his right hand which was successfully treated during the surgery.

He was to remain in hospital for three to four days under observation.

"Doing well & in great spirits!" Lippert tweeted on his Twitter account, saying he and his family were "deeply moved" by messages of sympathy and support.

"Will be back ASAP to advance US-ROK alliance," he added.

The US State Department condemned the "act of violence" and 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.

- Surprise attack -

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

"This man suddenly jumped out of his 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.

A Unification Ministry official told AFP that Kim had visited North Korea at least half a dozen times between 2006 and 2007.

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.

The annual exercises kicked off earlier this week, triggering a surge in tensions with the North.

Nearly 30,000 US troops are permanently stationed in South Korea and the United States would assume operational command in the event of an armed conflict with the North.

Lippert has proved a popular ambassador, tweeting regularly about his life in Seoul and setting up a tongue-in-cheek Twitter account for his dog, Grigsby.

His wife recently gave birth in Seoul and the couple gave their son a Korean middle name.Fac