Obafemi Martins' exit from the Seattle Sounders is all but official.

The Nigerian striker tweeted a farewell letter to Seattle Sounders fans Wednesday, confirming he will sign with Chinese side Shanghai Shenhua. News of Martins' impending move was reported earlier this week, and the 31-year-old was spotted arriving in South Korea on Wednesday for the club's preseason camp:

"For me it's now time for a new challenge," Martins wrote. "After significant reflection, I have decided to join Shanghai Greenland Shenhua FC of the CSL."

Martins leaves the Sounders after three campaigns in MLS, having recorded 40 goals and 23 assists over 72 regular season appearances. He enjoyed his finest season in 2014, notching 17 goals and 13 assists while earning a spot on the shortlist for MLS MVP.

Sounders general manager Garth Lagerwey said in an audio statement released by the team Sunday: ''Obafemi's not here with us in this phase of camp. We've had an approach from a Chinese club for his services.''

Information from The Associated Press was used in this report.