US Ambassador to South Korea Mark Lippert has been injured by an attacker in Seoul, officials and media say.
They said the attack happened as Mr Lippert, 42, was about to attend a breakfast meeting in the capital.
The YTN news channel posted a photo, showing the envoy bleeding from his head and left hand. He was reportedly cut by a razor blade.
Mr Lippert was taken to hospital. The attacker, whose identity was not immediately known, was arrested.
Media reports say the envoy's injuries are not life-threatening.
Assailant opposed military exercises - reports
The attack happened at about 11.40am (10.40pm GMT Wednesday, as the ambassador was entering a lecture hall in central Seoul, South Koran police were quoted as saying by the Yonhap news agency.
The assailant reportedly shouted "South and North Korea should be reunified!" before lashing out at the envoy.
The attacker also expressed his opposition to annual military exercises held jointly by South Korea and the US, which are currently underway.
US President Barack Obama later called Mr Lippert to wish him a speedy recovery, Reuters reported.
Mr Lippert, a former US assistant secretary of defence, was appointed US Ambassador to South Korea in 2014.