U.S. Amb. to South Korea Mark Lippert was in stable condition on Thursday after being cut on the face and wrist by a pro-North Korean activist who said he was against the ongoing military drills by Seoul and Washington, South Korean and U.S. officials said.

A knife-wielding assailant, identified as 55-year-old Kim Ki-jong, attacked the envoy who was having breakfast at an event in downtown Seoul where he was to give a lecture.

Lippert, bleeding, was rushed to Severance Hospital in western Seoul and underwent an operation, police said. Kim was arrested on the spot, with the police considering various charges for the warrant, including attempted murder.

The case was later taken over by prosecutors, who said they considered the violence an act of terror. A prosecution official who spoke on condition of anonymity said the anti-terrorism bureau, not the regular criminal bureau, within the Seoul Central District Prosecutors' Office would be in charge of the investigation.

"We considered the gravity of the issue and that the act could be seen as terrorism," the official said.

Lippert posted on his Twitter account after the surgery that he was "doing well and in good spirits" and that he was "deeply moved by the support" he received.

"Will be back ASAP (as soon as possible) to advance US-ROK alliance!" he wrote, using the acronym of South Korea's official name, the Republic of Korea.

The envoy sustained cuts to a total of five spots, including an 11-cm-long, 3-cm-deep gash extending from his right chin to cheek, Jung Nam-shik, head of the Yonsei University hospital, told reporters in a briefing. The envoy was also wounded on his left arm and fingers.

Jung said fortunately, there was no damage to Lippert's key facial organs, such as the nerves and salivary glands. There was, however, a minor wound to the nerves in his left pinky finger, he added.

Jung also said Lippert received more than 80 stitches and will need to remain hospitalized for three to four days.

Another hospital official said the ambassador was surprisingly calm despite worries from his family.

The suspect appeared to have broken his ankle during the incident, Yun Myeong-seong, head of the Jongno Police Station in Seoul, told reporters. Kim was interrogated for three hours on a stretcher before being transported to a hospital.

It is the first time a U.S. ambassador has been attacked in South Korea. Kim was also behind the first-ever assault against a foreign ambassador here, which took place in 2010. He was given a suspended jail term for throwing pieces of concrete at then Japanese envoy to Seoul Toshinori Shigeie. In a book detailing the incident published last year, Kim reportedly idealized terrorism.

South Korean intelligence sources said Kim had visited North Korea six times between 2006 and 2007. He also tried to erect a memorial altar in late 2011 for late North Korean leader Kim Jong-il in the heart of Seoul.

In 2007, Kim set himself on fire in front of the presidential office in Seoul, asking for an inquiry into a rape that allegedly took place at his office in 1988.

Lippert, 42, took office last year as the youngest-ever U.S. ambassador to Seoul. His wife gave birth to a son here in late January and the couple gave him a Korean middle name. He was formerly the assistant secretary of defense for Asian and Pacific security affairs from 2011 to 2012.

The suspect shouted his opposition to the annual Key Resolve and Foal Eagle military exercises that started Monday as he was being taken into a police car, authorities said. The exercises are part of Seoul and Washington's efforts to better deter threats from North Korea.

Kim is the head of a liberal organization that protests Japan's territorial claims over South Korea's easternmost islets of Dokdo.

According to recent blog posts, Kim appears to have shifted his attention from the islets issue to the role the U.S. plays in inter-Korean relations.

On Tuesday, Kim wrote a post that condemned the military drills between South Korea and the U.S., calling it "the reason why the reunion between family members (separated by the 1950-53 Korea War) couldn't take place."

Kim told reporters as he was about to get an X-ray that he had planned the attack for about 10 days. He said he was the one who "made the U.S. Embassy here create fences around it in 1985," when he cut and immolated the U.S. national flag after barging into the building.

"I (attacked the ambassador) because I didn't like how a moron who's barely in his 40s was going to take on our inter-Korean policy," he said.

Kim was also found to have been appointed by Seoul's unification minister twice to lead efforts to educate South Koreans on the need for reunification. Kim served two terms as a member of the unification education council from 2006 to 2009. There are more than 1,000 such members at home and abroad. The news sparked criticism as to whether the government conducted a thorough background check on the council members.

The organizer of Thursday's event, the Korea Council for Reconciliation and Cooperation, said Kim had not been invited to the event but somehow was able to get in.

Police said the U.S. envoy was not one of the personnel requiring 24-hour guarding. They also said the U.S. Embassy here had not requested a security escort.

In a statement by U.S. State Department deputy spokeswoman Marie Harf, the U.S. said it "strongly (condemns) this act of violence."

South Korean President Park Geun-hye also denounced the violence as an "attack on the alliance between South Korea and the U.S."

Seoul's foreign ministry said South Korea and the U.S. have agreed not to let the incident harm their alliance.

Curtis Scaparrotti, commander-in-chief of the South Korea-U.S Combined Forces Command, told reporters that the training will continue as scheduled.

Police said they will strengthen the security around U.S.-related facilities and personnel from now on to prevent similar attacks in a measure criticized as being too late.

Lippert will be provided with four police officers as guards, and his wife three officers, effective as of 10 a.m. Thursday, authorities said. (Yonhap)