South Korea's ruling and opposition parties expressed deep regret on Thursday over an attack on the U.S. ambassador to South Korea, denouncing it as an act of terrorism.

A 55-year-old progressive activist cut Mark Lippert's face and wrist with a knife as the U.S. envoy was preparing to give a lecture at a breakfast function in central Seoul. It is the first time a U.S. ambassador has been assaulted in South Korea.

"Terrorism against the U.S. ambassador to South Korea is tantamount to terrorism against the South Korea-U.S. alliance," Kim Moo-sung, chairman of the ruling Saenuri Party, said during a party leadership meeting.

"The authorities should make this a lesson, as any attempt to address a problem through violence cannot be accepted, by thoroughly investigating the case and strictly punishing those responsible for the attack," he said.

The main opposition New Politics Alliance for Democracy called it "an incident that should have not taken place."

"The attack on the diplomat that took place at the center of Seoul should not have happened," Yoo Yoon-keun, floor leader of the party, said during a party meeting. "Terrorism must not be pardoned in any case," he said. "The government should swiftly and precisely deal with the incident so it cannot affect the South Korea-U.S. alliance."

The party's chairman Moon Jae-in echoed that violence can never be justified.

"This appears to be an act of politically driven terrorism," he said during a meeting with South Chungcheong Governor Ahn Hee-jung in central Seoul.

"(Terrorism for political purposes) is not right in itself and always worsens problems ... This type of hatred is really undesirable," he said.

Na Kyung-won, chairman of the parliamentary committee on foreign affairs and unification, also expressed "deep regret" over the attack.

"A terrorist attack that should not have taken place happened to an envoy of a country," the lawmaker of the governing Saenuri Party, told Yonhap News Agency by phone.

She said the committee will consider officially conveying a message of consolation to Lippert or the U.S. government or having relevant government offices report on the case depending on the outcome of the upcoming probe.

"We'll decide on our additional response after watching the outcome of the investigation," she said.

South Korean civic groups also condemned the attack on the American envoy in unison.

"It's wrong for him to try to present his opinion through violence," said Kim Sam-soo, who is in charge of inter-Korean unification affairs at the Seoul-based Citizens' Coalition for Economic Justice. "We hope this incident will not make relations between South Korea and the U.S. uncomfortable and we pray for the rapid recovery of Ambassador Lippert."

The conservative Citizens United for a Better Society issued a commentary saying, "Violence and terrorism cannot be justified for any reason."

It urged the authority to "thoroughly investigate the terrorist and possible forces behind him and work out measures to prevent similar occurrences," the group said. 

According to police, the suspect shouted his opposition to the annual Key Resolve and Foal Eagle military exercises that started Monday and are part of Seoul and Washington's efforts to better deter threats from North Korea.

Some 28,500 U.S. troops are stationed in South Korea as a deterrent against North Korean provocations. The two Koreas remain technically in a state of war as the 1950-53 Korean War ended in a truce, not a peace treaty. (Yonhap)