The Sweden striker had submitted a transfer request a week ago, amidst interest from Liverpool.
While the team was on tour in Asia, Isak left their base in Newcastle to train in San Sebastian, Spain, at the facilities of his former club Real Sociedad.
He is accompanied by a personal trainer and has been cleared to train after a medical check-up.
Newcastle has not confirmed whether Isak's actions were taken with their permission, and the striker may face disciplinary action as a result.
