Southampton are weighing up a bid for unsettled Celtic central defender Virgil van Dijk, sources have told ESPN FC.

Saints boss Ronald Koeman is keen to add a central defender to his squad after missing out on the permanent signing of Toby Alderweireld, who joined Tottenham earlier this month.

While Celtic are aware of Southampton's interest in Van Dijk, the Scottish champions are keen to try to qualify for the Champions League group stages before any potential deal is done.

The Dutch defender has made it clear that he is keen to leave this summer, and has encouraged his club to consider Premier League offers.

"Of course the Champions League millions are important for Celtic, but they cannot dismiss any interest," he told Voetbal International.

"If the correct amount is placed on the table, then they will really think about it. If Celtic want to earn money from me, the club must act now.

"It is time for a new challenge. This year I'll turn 24, and I need something new. More pressure, better opponents, especially when you think of the European Championships next year.

"It's my dream and my goal to play for Holland. I keep setting targets and my aim is to make it as difficult as possible for the national coach to ignore me.

"A transfer to the English Premier League would help, but I'm open to all top leagues -- England, Germany, Spain, France.

"But in the end I have to like the team and Celtic has to agree with the bid and transfer."