The rumoured Newcastle United target has scored 11 goals so far this campaign, including the winner against Dagenham & Redbridge at the weekend.
Roofe, who was let go by West Bromwich Albion in the summer, was said to have been the subject of an enquiry from a top-flight club.
Ashton has now moved to clarify the situation, however, claiming that Oxford are "under no pressure" to sell their star assets in their bid to escape League Two.
"Nobody has called directly on Kemar Roofe or about any of our players," he told BBC Sport. "There's lots of chatter in the background from agents, but that's what the industry is like."
"Over the last six to nine months, the type of clubs attending [the Kassam Stadium] has changed dramatically. But, myself, chairman Darryl Eales and Michael Appleton are under no pressure to sell players. We want to keep this squad together and our key focus is to get us out of this division."
Oxford are currently two points adrift of league leaders Plymouth Argyle, while also sitting four points ahead of fourth-place Accrington Stanley.