SEOUL (AFP) - United States Ambassador to South Korea Mark Lippert was recovering from surgery on Thursday following a knife attack that slashed open his cheek and damaged nerves in his right hand, doctors said.

Mr Lippert, whose condition was described as stable, was treated for 21/2 hours by a plastic surgeon and an orthopaedic surgeon and will remain in hospital under observation for the next three to four days.

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

Mr Lippert said he was well and in great spirits as he recovered from surgery. "Doing well and in great spirits! Robyn, Sejun, Grigsby and I - deeply moved by the support!" Mr Lippert tweeted, referring respectively to his wife, newborn son, and dog.

"Will be back ASAP to advance US-ROK alliance!" he said, referring to the Republic of Korea.

Mr Lippert was attacked on Thursday morning during a breakfast function by a knife-wielding political activist who struck at his face, lower arm and right hand. One of the surgeons, Dr 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 3cm 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.