Isak had submitted a transfer request over a week ago and had been training in San Sebastian, Spain, at the facilities of his former club, Real Sociedad.
Despite a £110m bid from Liverpool being rejected by Newcastle in the past 48 hours, the club remains firm in their stance that Isak will only be sold for £150m or more.
Newcastle's manager, Eddie Howe, has admitted that he was only learning about Isak's whereabouts through the press, indicating a potential strain in their relationship.
In a separate move, Newcastle has made an offer of £70m to RB Leipzig for Slovenian striker Benjamin Sesko, who could potentially replace Isak at the club.
