Lippert, 42, was bleeding from a facial wound but was walking after the attack as he was taken to the hospital. He was later reported to be in stable condition and officials in Washington said his injuries were not life-threatening.
The assailant was caught by police and identified as 55-year-old Kim Ki-jong. In 2010, Kim tried to attack the Japanese ambassador to Seoul by throwing a piece of concrete and was given a suspended jail term, according to police.
Witnesses and police said Kim used a small fruit knife in the attack, which took place inside a large government arts center across the street from the heavily guarded U.S. embassy on Seoul’s main ceremonial thoroughfare.
“We strongly condemn this act of violence,” U.S. State Department spokeswoman Marie Harf said.
U.S. President Barack Obama quickly called Lippert to wish him a speedy recovery, a White House official said.
The assailant was dressed in traditional Korean clothing and shouted that North and South Korea should be reunited just before he attacked Lippert. He also shouted that he opposed “war exercises”, a reference to annual joint U.S.-South Korean military exercises that began this week.
“I carried out an act of terror,” Kim shouted as he was pinned to the floor by event attendees.
Kim said while in police custody he had acted alone. South Korea’s Yonhap news agency reported that Kim also said he was part of a group that had cut and burned a U.S. flag on the embassy grounds in Seoul in 1985.
Kim is a member of the pro-Korean unification group that hosted the event, police said. He also stages one-man protests against Japan over disputed islands known as Dokdo in Korean and Takeshima in Japanese.
“The guy comes in wearing traditional Korean brown and tan dress. He yells something, goes up to the ambassador and slashes him in the face,” witness Michael Lammbrau of the Arirang Institute think tank told Reuters.