The 55-year-old attacker reportedly also opposed the annual military drills conducted jointly by South Korea and the United States and condemned by Pyongyang as a rehearsal for war.
According to the broadcaster, there is no evidence the assailant was a North Korean agent.
The man, having thrown concrete at the Japanese ambassador to South Korea in 2010, has a history of militant Korean nationalistic activism, the BBC reported.
The attack on US Ambassador Mark Lippert was confirmed by the US State Department earlier on Thursday. The agency strongly condemned the assault.
In the attack, Lippert's face and left hand were slashed. The ambassador is being treated at a local hospital. His injuries are not life-threatening.
Mark Lippert, who formerly served as the chief of staff to the outgoing US secretary of defense, Chuck Hagel, has been US ambassador to South Korea since October 20, 2014.
