The Aussie striker's agent Andrea D'Amico said £10m-rated Viduka would only be lured away from Leeds by a high-profile club.
He told BBC Sport: "We are still working on the deal because it is an important one for the club and the player.
"If Mark leaves Leeds, it will only be for an important club in Europe, but it would be better to talk about which club after the event.
"At the moment the market is difficult, but if the right deal comes up it is possible Mark will leave."
D'Amico claimed Chelsea have joined Barcelona, AC Milan and Lyon in tracking Viduka, who wants to leave Leeds before the transfer window closes on 31 August.
However, the host of 'interested' clubs identified by D'Amico have been notable only by their silence.
Asked whether Viduka would make his desire to leave Leeds official by handing in a transfer request, he said: "At the moment he is with Leeds, training and preparing for the new season, that is all that should be said."
No doubt plenty more will be said in the next few weeks.