The knife attack on the U.S. ambassador to Seoul was carried out by pro-North Korea forces, South Korea's government and ruling party said Friday, vowing to conduct a thorough investigation into the case.

High-level officials from the government, presidential office and ruling Saenuri Party came to the conclusion during a meeting at the Prime Minister's Office, one day after Amb. Mark Lippert sustained cuts to his face and wrist in a knife attack by a South Korean activist who voiced his opposition to joint military drills with the U.S. The exercises are part of Seoul and Washington's efforts to better deter threats from North Korea.

A preliminary probe showed that the assailant, Kim Ki-jong, had been to North Korea seven times between 1999 and 2007. He also tried to erect an altar at the heart of Seoul in memory of late North Korean leader Kim Jong-il in December 2011 shortly after he died.

"We defined this case as one carried out by pro-North forces and agreed to thoroughly determine the truth and who was behind it," Rep. Yoo Seong-min, the floor leader of the ruling party, said in a press briefing following the meeting that was also attended by Prime Minister Lee One-koo; President Park Geun-hye's chief of staff, Lee Byung-kee; and Saenuri Party Chairman Kim Moo-sung, among others.

The officials also agreed to make joint efforts to ensure the incident causes no harm to the South Korea-U.S. alliance and beef up security for foreign missions here, Yoo said.

With enough evidence, the assailant could be charged with violating South Korea's National Security Law, which bans South Koreans from publicly sympathizing with the North Korean government, according to local authorities.

The main opposition party New Politics Alliance for Democracy (NPAD) also condemned the attack, calling it an "unprecedented act of terror" and an "unpardonable serious crime."

The party, however, expressed caution against exaggerating the incident or using it for political gain.

"Some people's excessive claim that (Kim) attacked the South Korea-U.S. alliance is inappropriate," Rep. Jung Chung-rai of NPAD said during a party meeting.

In South Korea, conservative parties such as Park's Saenuri Party have often stirred up anti-North Korean sentiment to rally support.

On Thursday, Park, who is on a four-nation tour of the Middle East, called the incident an attack on the South Korea-U.S. alliance. (Yonhap)