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The 22-year-old returned to Villa Park this week after an impressive three-month loan spell at Sheffield United.

Blades boss Bryan Robson has already indicated his desire to secure the England Under 21 international's signature in a £2million deal in January.

But O'Neill has confirmed United could face a real fight to secure Cahill's services with interest coming from a host of clubs, believed to include Birmingham, Wigan and Derby.

"There has been a substantial amount of interest in him and I think Gary is aware of that because people have been speaking to his agent," O'Neill told the Yorkshire Post.

"I will sit down and speak to him to run through things with him."

O'Neill has already said he would not make a decision on Cahill's future until he has resolved contract negotiations with defenders Olof Mellberg and Martin Laursen.

Cahill, who is set to sit out the Christmas period after undergoing surgery on a facial injury, has already stressed his desire to play regular first-team football.

Birmingham, Wigan and Derby are all looking to strengthen next month and can offer Premier League football but Cahill may opt to stay at Bramall Lane and further his career with the ambitious Championship club.