South Korea's police say they've arrested a 33-year-old jobless man for allegedly making an online threat to kill the U.S. ambassador in Seoul.

Police said Friday the man allegedly left a English message vowing to "kill" Mark Lippert with "nuclear poisoning" on the website of the White House on July 8.

The man, surnamed Lee, has denied the allegation. But police say they've found a draft of the online threat from his laptop computer.

Police say the message didn't clearly explain why he wants to kill Lippert.

In March, Lippert was attacked by another man who slashed him with a knife during a forum. Lippert suffered deep gashes on his face and arm and was treated at a hospital for five days.

The assailant was charged with attempted murder.