Norwegian world chess champion Magnus Carlsen has confirmed that he pulled out of a recent tournament and quit during a match against Hans Niemann because he believes the 19-year-old American has been cheating.

Carlsen last week sent shockwaves across the chess world when he resigned after just one move during a match against Niemann in the Julius Baer Generation Cup. The move came just one week after the 31-year-old grandmaster withdrew from the Sinquefield Cup in the U.S. after losing to Niemann, who has admitted to cheating in online games in the past. Many speculated Carlsen's abrupt withdrawal was connected to the ongoing alleged cheating scandal, although he had initially refused to say so.

On Monday, Carlsen confirmed in a statement that he made the "unprecedented" decisions because he believed that Niemann's cheating goes beyond what the American has already admitted to.

"I know that my actions have frustrated many in the chess community," Carlsen wrote. "I'm frustrated. I want to play chess. I want to continue to play chess at the highest level in the best events."

"I believe that cheating in chess is a big deal and an existential threat to the game," he continued. "I also believe that chess organizers and all those who care about the sanctity of the game we love should seriously consider increasing security measures and methods of cheat detection for over the board chess."

Niemann had previously admitted he had not played fairly in non-competitive games on chess.com when he was younger, Reuters reported. But he has denied any cheating while playing live games.

After Carlsen resigned from the tournament, Sinquefield Cup chief arbiter Chris Bird said there had been "no indication that any player has been playing unfairly," according to Reuters.

But Carlsen on Monday said that he believes that Niemann has cheated more than previously disclosed and said his demeanor during their match in the Sinquefield Cup confirmed his suspicions.

"I believe that Niemann has cheated more — and more recently — than he has publicly admitted," Carlsen wrote. "His over the board progress has been unusual, and throughout our game in the Sinquefield Cup I had the impression that he wasn't tense or even fully concentrating on the game in critical positions, while outplaying me as black in a way I think only a handful of players can do. This game contributed to changing my perspective."

Carlsen alluded to their being more information he would like to share, but said he is "limited" in what he can reveal "without explicit permission from Niemann to speak openly."

"We must do something about cheating, and for my part going forward, I don't want to play against people that have cheated repeatedly in the past, because I don't know what they are capable of doing in the future," he said.