K-pop group SHINee on Tuesday mourned the death of lead singer Kim Jong-hyun, who reportedly took his own life, on social media.

Kim, better known by his stage name Jonghyun, died in southern Seoul on Monday. He was rushed to a hospital in a state of cardiac arrest but was later pronounced dead. The singer left behind a painful note describing his harrowing fight against depression.

"Jonghyun is the best artist who loves music, enjoys the stage and cherishes communicating with fans through music," the group said in a statement issued on Twitter next to an image of Kim performing. The statement ended by saying, "We will remember you forever."

Thousands of fans at home and abroad also paid respects to the late idol star on the social media platform.

"Thank you for everything. You did so incredibly well," wrote one fan with the Twitter handle @MaryMyHeart. Another user, @wizjimin, said "Rest in Peace my angel, we love you so much."

S.M. Entertainment, management agency of SHINee, issued a statement on Kim's death late Monday.

"Members of SHINee who have been together for a long time, fellow S.M. artists and the staff are mourning the deceased in huge shock and sadness," the company said.

A private memorial for Kim was set up at Asan Medical Center in eastern Seoul. (Yonhap)