Mack, who hasn't reported to training camp, enters the final year of his contract on a fifth-year option, which pays a base salary of $13.8 million.
The Raiders, however, haven't come forward with a present offer and there have been no contract discussions with Mack's agent, Joel Segal, since February, NFL Network Insider Ian Rapoport reported Tuesday.
The Raiders hope to improve defensively under coach Jon Gruden, but the unit simply isn't the same without Mack.
Over the past four seasons, Mack has developed into one of the NFL's elite pass rushers, totaling 40.5 sacks over the past four seasons. Mack, a three-time Pro Bowler, has garnered first-team All-Pro selections twice (2015-16) and was also named the NFL's Defensive Player of the Year in 2016.
In the meantime, Rapoport previously categorized the discussions as "tense" and it appears there won't be any resolution any time soon.
"It's unfortunate that we have to go through this," Raiders general manager Reggie McKenzie said, via Vic Tafur of The Athletic. "But his camp decided to do it this way ... and Khalil is not the first guy to hold out. But we'll get through this. But there is nothing really to report. He is going to hold out until he gets an extension, and that's where we are."
With each missed day of training camp, Mack is subject to a $30,000 fine under the fifth-year option as opposed to a $40,000 fine, according to the collective bargaining agreement.
NFL Network's Steve Wyche previously reported in April that Mack seeks a new deal with guarantees in excess of $65 million.