The club's Saudi owners are said to have told Isak that he will not be allowed to leave during the summer, regardless of whether they recruit another striker or not.
The message has created great tensions between the Swede and the club, reports the British site.
Isak has pushed for a transfer to Liverpool and has missed training matches and Newcastle's preseason tour in Asia during the summer.
The club's coach Eddie Howe has now stopped Isak from training with the team, and the 25-year-old will have to continue training on his own until he accepts the situation, writes The Telegraph.
Isak's contract with Newcastle runs until 2028.
