Aberdeen have rejected a bid in the region of £3.5m from Celtic for defender Scott McKenna.
The Pittodrie club are adamant that the 21-year-old centre-back will not be sold during this transfer window.
The Scotland international, who is currently recovering from a hamstring injury, has won four international caps since making his debut in March.
Celtic are keen to strengthen their defensive options, with Dedryck Boyata's future still uncertain.
Aberdeen rejected three bids from Hull City in January for McKenna, who signed a contract extension on improved terms in March this year, running to 2023.
Swansea were also believed to be interested in the player before the English transfer window closed earlier this month.
McKenna played in both legs of Aberdeen's Europa League tie with Burnley but suffered a hamstring injury in their opening Premiership game against Rangers, which will keep him out until next month.
'He's one for the future' - analysis
Former Celtic forward Kris Commons on BBC Radio Scotland's Sportsound
He would be a good addition because Celtic have been crying out for a good centre-half.
He's not had a great deal of football - four international caps - so at the minute he's one for the future. In 12-18 months you'll probably see the best of him. He's on a long-term deal as well so he's in no rush.
Celtic aren't going to pay £8m, £9m, £10m for a centre-half that's 21 and was playing on loan at Ayr United just over a year ago. He got his experience in the lower leagues but this is a guy that's only played 30-plus games for Aberdeen. He's not played 300 games in competitive football.