Aberdeen have confirmed striker Duk is absent from the club without permission and is now the subject of internal disciplinary proceedings.
The Cape Verde international has failed to report for training.
Aberdeen have attempted to communicate with the absent striker but state Duk “has refused to engage with us directly.”
The Dons are “disappointed” the matter has progressed to that level.
Duk has failed to join the Dons for pre-season – with the first game of the campaign on Saturday against Queen of the South in the Premier Sports Cup.
The attacker also did not attend the Dons’ warm weather camp in Portugal last week, with a “medical issue” cited by boss Jimmy Thelin at the time.
The 24-year-old missed the first week of pre-season training in the Granite City under new boss Thelin before the Algarve trip because he had been on international duty during the summer.
Duk recently confirmed he wants to leave Aberdeen and urged the club cash in on him by selling him this summer.
Attacker Duk is in the final year of his deal with the Dons and is free to sign a pre-contract with another club on January 1.
A club spokesperson said: “Duk has failed to report to training and is currently absent without permission.
“The club has made attempts to communicate with Duk but he has refused to engage with us directly.
“He is now subject to internal disciplinary proceedings.
“We’re naturally disappointed with the situation but can’t say anything further at this stage.”
‘We have some internal issues with the Duk situation’
In his recent comments, Duk claimed he had informed Aberdeen of his decision to move on this summer and added he hoped to land a switch to one of the major leagues in Europe.
The attacker’s agent Carlos Oliveira also urged Aberdeen to grant Duk’s request to be sold this summer.
Duk’s former club Benfica are reportedly due 50% of any transfer fee.
Speaking at his pre-Queen of the South media conference, Aberdeen boss Thelin said: “Duk is not going to be in the squad (to face Queen of the South).
“Right now we have some internal issues with the Duk situation, that is all I can say about that.”
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