Borussia Dortmund striker Sebastien Haller said he will undergo further surgery as part of ongoing treatment for a testicular tumour that has kept the player sidelined.

The 28-year-old Ivory Coast forward had surgery earlier this year after discovering the tumour while at a training camp in July, and has been undergoing chemotherapy in recent months. Haller, who previously played at English Premier League club West Ham United, briefly returned to training with former team Ajax Amsterdam last month.

"As planned from the beginning, different possibilities were considered following the chemo. I tell you that the fight is not over for me," Haller wrote in a Twitter post on Wednesday.

"I will have to undergo an operation to permanently end this tumour which keeps me away from the pitch. I want to thank you all for the messages of support and the strength you give me in this ordeal. Looking forward to seeing you all soon."

German team Dortmund signed Haller for an initial fee of 31 million euros ($32.18 million) in the close season after the striker scored 34 goals last season for Dutch champions Ajax - including 21 in the league and 11 in the Champions League. ($1 = 0.9632 euros)