The 35-year-old forward is currently without a club after his contract with Italian giants Napoli expires at the end of last month.
Mertens spent the last nine years with the Serie A outfit, scoring 148 goals in 397 appearances, and journalist Fabrizio Romano claims that he has turned down a contract offer to remain at the club.
Italian news outlet Il Mattino, however, reports that the Belgian is open to staying with Napoli and will only remain in Italy if he agrees a new deal with the Azzurri.
The report adds that both Leeds and Newcastle are closely monitoring Mertens's situation, while Ligue 1 club Marseille have also registered their interest in the experienced forward.
Mertens is believed to be training in Ibiza at present to keep himself in shape ahead of a potential move to a new club before the 2022-23 season begins.
Leeds head coach Jesse Marsch has publicly revealed that the club are "actively hunting a striker" this summer, while Newcastle may now turn their attention to Mertens as negotiations with top target Hugo Ekitike have stalled.