Middlesbrough need a proven goalscorer at The Riverside before they compete in the Uefa Cup for the first time in their history next season and Viduka, 28, remains their leading target.
Viduka's commitment to Leeds has been questioned recently and he let the team down when they needed him most against Bolton on Sunday.
That will not put off Boro boss Steve McClaren from bidding around £5m, as he tried to sign the Aussie in the transfer window last January.