The US says it will not be intimidated by anyone who threatens diplomats, and attackers will be prosecuted.
The warning has been issued following a knife attack on the American ambassador to South Korea.
Mark Lippert's cheek and arm were slashed in the capital, Seoul. Pictures showed him leaving an event covered in blood.
Mr Lippert tweeted to say he is doing well after the attack:
Doing well&in great spirits! Robyn, Sejun, Grigsby & I - deeply moved by the support! Will be back ASAP to advance US-ROK alliance! ê°™ì´ ê°‘ì‹œë‹¤!
— Mark Lippert (@mwlippert) March 5, 2015
President Barack Obama called his former aide Mark Lippert to wish him a swift recovery after the assault during a breakfast speech in Seoul.
The 42-year-old envoy was taken to hospital where he underwent surgery for more than two hours, but his injuries were not life-threatening.
He received an 11cm wound to the right of his face requiring 80 stitches and a cut to his left arm which ruptured a tendon and caused nerve damage.
The ambassador was said to be calm and composed throughout the procedure.
A State Department spokeswoman said: "We strongly condemn this act of violence."
Footage taken in the aftermath of the attack, showed the ambassador being rushed out of the building holding one hand to his bleeding right cheek, with his other hand smeared with blood.
Assailant
Security staff and police officers were seen jumping on the ambassador's assailant who was armed with a 10-inch blade.
Police have identified the suspected attacker as 55-year-old Kim Ki-Jong, who has a previous conviction for assaulting the Japanese ambassador to Seoul in 2010.
"I carried out an act of terror," Kim shouted as he was pinned to the floor.
District police chief Yoon Myung-Soon said: "We have detained him and are investigating the cause of the attack and other circumstances."
A spokesman for the Korea Council for Reconciliation and Cooperation, which hosted the event, apologised for the lack of security.
Mr Lippert, a long-time adviser to President Obama and former US assistant secretary of defense for Asian affairs, only took up his post in South Korea last October.
His wife recently gave birth to their son, to whom they gave a Korean middle name.
Mr Lippert has previously served as a intelligence officer for naval special operations, and won a Bronze Star Medal following a tour in Iraq.
South and North Korea have been divided since the 1950-53 Korean War and are still technically at war because the fighting ended in a truce.
The US and South Korea launched annual joint military exercises this week leading to heightened tensions with the communist North.
Pyongyang claims they are rehearsals for an invasion, while South Korea and the US argue they are purely defensive.
Following the attack on the ambassador, a South Korean defence ministry spokesman said the exercises would continue as planned.
America has almost 30,000 troops permanently stationed in the South.
Originally posted at 6.55am
