A South Korean court on Friday handed down a 20-year jail sentence on a US citizen charged while still a teenager with brutally murdering a Korean man nearly two decades ago.

Arthur Patterson, now 36, was extradited from the United States in September last year to face trial in Seoul on charges of murdering Jo Jung-Pil, who was stabbed multiple times with a knife in a fast-food restaurant toilet.

Patterson, who was the 17-year-old son of a US military contractor at the time, had denied any involvement in the crime in the nightlife district of Itaewon, close to the US military base in Seoul.

Prosecutors, however, argued that Patterson had shown a "devil-like brutality" in stabbing Jo, who appeared to be a complete stranger to him, to death for no apparent reason.

Twenty years in jail is a maximum penalty in South Korea for an offender under the age of 18.

Patterson was initially tried in 1997 as an accomplice, while Edward Lee, a Korean-American man who was at the scene, was charged with murder. Both accused the other of killing Jo.

Patterson served 18 months in prison for lesser charges including obstruction of justice, while Lee was sentenced to 20 years in jail for murder.

In 1998, Patterson was released as part of an amnesty programme, only to find himself a murder suspect again after Lee was acquitted on appeal for lack of evidence.

He fled to the United States a year later after investigators failed to renew his travel ban -- a mistake that sparked a storm of criticism.

The murder was made into a hit movie in 2009, refuelling public anger over the crime and forcing South Korean prosecutors to reopen the case.

Patterson was formally charged in absentia with Jo's murder in 2011, and extradition proceedings began.