"I understand the debate about Radja Nainggolan. It was difficult for myself and the staff. I'm not here to make popular decisions, but the right decisions. Belgian Football has a lot of good players," Martinez said as quoted by the team's official Twitter account.

Soon after learning he was not included in Belguim's World Cup squad, 30 year-old Nainggolan announced his retirement from international football. He had made a total of 30 international appearances since his senior debut for Belgium in 2009, scoring six goals.

Russia will host its first-ever FIFA World Cup from June 14 to July 15, with the matches set tot take place in 11 cities across the country.

Belgium will begin their World Cup campaign with a game against Panama on June 18 before facing Tunisia five days later. They will play England in their final Group G game on June 28.