Chess superstar Magnus Carlsen for the first time openly accused American Hans Niemann of cheating Monday, saying the rising star had done so more recently and more often than he previously admitted.

The chess world has been shaken for three weeks by the Carlsen-Niemann controversy, since the 31-year-old five-time world champion withdrew from the Sinquefield Cup in the US after losing to Niemann, 19.

Last week, the Norwegian's abrupt withdrawal from a match against Niemann in the Julius Baer Generation Cup online tournament reignited the furore.

In a letter published on Twitter and addressed "Dear Chess World," Carlsen said: "I believe that Niemann has cheated more -- and more recently -- than he has publicly admitted.

"His over the board progress has been unusual."

He did not specify the exact form of the alleged cheating, but added there was "more that I would like to say".

Niemann has acknowledged cheating online twice, when he was 12 and 16, but says he has never played fraudulently in a face-to-face match and is even willing to play nude to prove his good faith.

In early September, the huge chess platform chess.com banned the American for cheating on the site.

AFP contacted Niemann for comment but he did not immediately reply.

During their Sinquefield Cup match, Niemann did not seem tense or even to be "fully concentrating on the game in critical positions, while outplaying me as a black in a way I think only a few players can do", Carlsen continued in his letter.

He called cheating in chess "an existential threat to the game" and said: "I don't want to play against people that have cheated repeatedly in the past."

He concluded by saying he hoped the truth would come out, "whatever it may be".