The man behind a shocking knife attack on the US ambassador to South Korea was convicted of attempted murder on Friday and sentenced to 12 years for an assault that left the envoy needing 80 stitches to a deep gash on his face.

The man behind a shocking knife attack on the US ambassador to South Korea was convicted of attempted murder on Friday and sentenced to 12 years for an assault that left the envoy needing 80 stitches to a deep gash on his face.

Prosecutors had sought a 15-year sentence for Kim Ki-Jong, 56, who lunged at Mark Lippert with a paring knife during a breakfast function in Seoul in March.

In passing judgment, the Seoul Central District Court noted Kim had "shown no repentance, attempting to justify his actions throughout the trial".

The public attack on the envoy of South Korea's most important military and diplomatic ally -- and dramatic TV footage of Lippert being rushed to hospital clutching his bloodied face -- was shocking for a country where passionate political protest is common but acts of such extreme violence rare.

There was also a degree of shame and disquiet that a man with a record of violence against foreign envoys had been able to carry out such an assault.

A maverick activist who was known to police, Kim had been handed a two-year suspended sentence in 2010 for hurling a rock at the then Japanese ambassador to Seoul.