A South Korean man was sentenced to 18 months in prison Friday for posting letters on the White House website, threatening to kill the top US envoy to Seoul and rape the second daughter of US President Barack Obama.

The Seoul Central District Court found the 34-year-old defendant, identified only by his surname Lee, guilty of attempted intimidation for posting a letter titled "Declaration Terror to Mr. President Obama" on the White House webpage for civil petitions in July last year.

In the letter, Lee said, "My assassinator's mind is too weak to cut the ambassador's artery perfectly," referring to an attack on the US Ambassador to South Korea Mark Lippert, which took place earlier in March 2015.

He was attacked with a knife at a breakfast function in Seoul by another South Korean man, leaving him with deep gashes on his face and arm that needed more than 80 stitches.

Lee was also found to have posted another letter, threatening to rape Sasha Obama, the second daughter of President Obama.

"The US Embassy took his letters as a serious threat against the United States government and demanded a thorough investigation and punishment to the local law enforcement authorities," the court said.

Still, the court found the defendant guilty of attempted intimidation, instead of intimidation charges, saying there is not enough proof to acknowledge that the letters were actually delivered to the victims.

Prosecutors had originally charged Lee with intimidation. (Yonhap)