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North Korean state media called the knifing of the American ambassador to South Korea "punishment" as the U.S. Embassy in Seoul beefed up security following the attack on Mark Lippert.

Lippert underwent a three-hour surgery after suffering wounds to his face and arm. A four-inch-long gash down the right side of his face required 80 stitches, hospital officials said.

The diplomat said he was "doing well and in great spirits" following the attack, writing in a Twitter post that he "will be back ASAP."

A hospital official told reporters that "some paralysis of his face and left arm senses are expected."

South Korean police said that the suspect shouted that the rival Koreas should be unified as he lunged at the ambassador. Police are considering whether to charge 55-year-old Kim Ki-jong with attempted homicide, a police official involved in the case told Reuters.

The attack was a protest against joint military exercises by South Korean and U.S. troops, which Kim said interfered with reconciliation between North and South Korea, police said following an interrogation.

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"I carried out an act of terror," Kim shouted as he was pinned to the floor by people at the event.

Witness Michael Lammbrau told Reuters that the suspect yelled something that "sounded like he was anti-American, anti-imperialist."

Lammbrau added: "The ambassador fought him from his seat ... There was a trail of blood behind him."

North Korea's official KCNA news agency called the attack "deserved punishment" and "the knife of justice" for U.S. military exercises with South Korea.

Witnesses and police said Kim used a small fruit knife in the attack, which took place in a government arts center across the street from the heavily guarded U.S. Embassy on the South Korean capital's main ceremonial thoroughfare.

South Korean police are providing additional security to the U.S. Embassy following the attack, an embassy spokeswoman told NBC News.

Kim visited North Korea eight times from 2006 to 2007, according to a South Korean Ministry of Unification official.

South Korea's Yonhap news agency reported that Kim also told police he was part of a group that had cut and burned a U.S. flag on the embassy grounds in Seoul in 1985.

A former Senate staff member for President Barack Obama, Lippert was a key adviser during the 2008 campaign, former chief of staff on Obama's National Security Council, and was confirmed as ambassador last September after serving under former Secretary of Defense Chuck Hagel in the Pentagon.

Known for his open, informal style, Lippert is active on Twitter and can often be seen walking his basset hound, Grigsby, in Seoul. His wife recently gave birth to a son, who was given a Korean middle name.

NBC News' Hasani Gittens and Cassandra Vinograd and Reuters contributed to this report.