Tottenham Hotspur must raise their offer for Vincent Janssen if they are to prise him away from AZ Alkmaar, the Dutch club have reiterated. AZ have taken advice from 'Moneyball' expert Billy Beane over the valuation of last season's Eredivisie top scorer, who netted 27 goals last term.

The Daily Telegraph understand that AZ are demanding £16.5m for Janssen and rejected an offer from Spurs of around £15m. American Beane, who joined the club as an advisor in March 2015, has calculated the worth of the 22-year-old who is openly frustrated over the delay.

AZ's technical director has already indicated that Tottenham's offer did not reach the club's expectations but has now delivered further detail over the reasons for their stubbornness. Janssen has a contract at the AFAS Stadion for another three years and AZ are ready to hold out for the player's true value.

"We have done extensive research to approximate the market value of Vincent, by people who understand it, for example," Huiberts told AD. "We have ambition at AZ and want to belong to the top four in the Netherlands. Then it does not help if we sell our players. Vincent here a year ago signed a contract for three years. Basically, we do not want to lose him."

While Tottenham are being left frustrated by the stalemate in negotiations, Janssen has risked punishment from the club after criticising their tactics in the transfer window. The former Almere City hitman wants a resolution to discussions so he can complete his switch to the Premier League.

"I have expressed my choice at the club," he was quoted as saying by Sky Sports. "Tottenham Hotspur have made a high bid in my eyes. There has been an offer around €18m (£14.9m), but AZ will not even come to the table.

"I would like somewhere else as soon as possible to start my preparation. This is frustrating. I indicated that I would take the next step and want to get away from the club. They know [this] and are aware of [the situation].

"It's a shame, I would have wished that it would go a little faster. Sure it's fine to train again. AZ is a fantastic club, but I want to make that next step for my development. Ultimately I hope to shake people's hands here and walk out of the door on good terms."