The 30-year-old is free to discuss terms with other parties with his contact due to expire at the beginning of July, with the striker understood to be keen on a move to the former Scottish Premier League outfit.
"All we are doing is registering a note of interest in players and Jon is one of a number who are out of contract with their own clubs at the end of this season. We're just confirming that status with the clubs," McCoist told RangersTV.
"There is nothing more to tell, nothing more to say but we are looking to bring players to the club and they have to be free transfers. We aren't able to register them until September 1 but he is one of a number we are looking at.
"He ticks a lot of boxes but unfortunately at this moment in time he ticks them for Dundee United. Time will tell whether we keep our interest in players like John but we are certainly looking to strengthen the squad for next year, that goes without saying."
Despite being under a transfer embargo, Rangers can offer contracts to free agents.