Security personnel detain an unidentified assailant who attacked the US ambassador to South Korea Mark Lippert at a public forum, in central Seoul March 5, 2015. [Photo/Agencies]

SEOUL - US ambassador to South Korea Mark Lippert was slashed in the face by a Korean nationalist but was not seriously hurt during an attack at a breakfast forum held in the capital on Thursday to discuss Korean reunification, police and witnesses said.

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.

"We strongly condemn this act of violence," US State Department spokeswoman Marie Harf said.

US President Barack Obama quickly called Lippert to wish him a speedy recovery, a White House official said.

South Korean President Park Geun-hye, speaking in the United Arab Emirates, called it an "attack on the South Korea-US alliance."

The assailant was caught by police and identified as 55-year-old Kim Ki-jong. Witnesses and police said he used a small fruit knife in the attack.

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.

Kim 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", an apparent reference to annual joint US-South Korean military exercises that began this week.