US ambassador to South Korea Mark Lippert was slashed in the face Thursday by a Korean nationalist, Reuters reported, citing police and witnesses.
Lippert, 42, was bleeding from a facial wound but was walking as he was taken to hospital.
He was later reported to be in stable condition, and officials in Washington said his injuries were not life-threatening.
The attack took place at a breakfast forum in Seoul to discuss Korean reunification.
Police caught the assailant, who used a small fruit knife, and identified him as 55-year-old Kim Ki-jong.
Kim was dressed in traditional Korean clothing. Just before he attacked Lippert, he shouted that North and South Korea should be reunited.
He also shouted that he opposed “war exercises”, an apparent reference to the annual joint US-South Korean military exercises that began this week.
Kim is a member of the the Korean Council for Reconciliation and Cooperation, the pro-unification group that hosted the event, Reuters said.
The group later issued a statement in which it condemned the attack and apologized to the governments of the United States and South Korea.
US President Barack Obama called Lippert to wish him a speedy recovery, a White House official said.
In 2010, Kim tried to attack the Japanese ambassador to Seoul by throwing a piece of concrete at him and was given a suspended jail term, police said.
He also stages one-man protests against Japan over disputed islands known as Dokdo in Korean and Takeshima in Japanese.
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