Sky Sports says Newcastle owner Mike Ashley is willing to break the club's 12-year transfer record of £16m for the Danish international, but the Dutch side are looking for more than £20m.
The 27-year-old has scored 26 goals in 43 games for Feyenoord since joining the club in 2016 from Copenhagen and has 28 caps for his country.
Meanwhile, Norwich City youngster Ben Godfrey is interesting Newcastle.
Godfrey is being watched by a host of Premier League scouts during his loan spell at League One Shrewsbury. Godfrey has also been watched by England Under-20 boss Paul Simpson, with a view to handing the 20-year-old a call-up to his World Cup-winning squad.