Chelsea have not given up on signing Kalidou Koulibaly despite Napoli turning down an offer worth up to £42million.

The 25-year-old centre-half is the club’s first choice to strengthen Chelsea’s defensive options this summer.

Koulibaly has made it clear to Napoli that he is not interested in a new contract and is keen to move to Stamford Bridge.

Chelsea are prepared to offer the Senegal international a five-year deal worth more than £60,000-a-week.

Napoli are doing their utmost to keep the defender and continue to insist that he is not for sale at any price.

So far the Blues have made a bid of about £37m with a series of performance related add-ons taking the fee up by another £5m.

However, they have been given hope that Napoli are resigned to losing their prized asset as they appear to be chasing a replacement themselves.

Napoli are looking at Torino defender Nikola Maksimovic and have had an opening offer of just over £21m rejected.

Chelsea have a lack of defenders and are vulnerable should one get an injury. At the moment coach Antonio Conte has just veteran captain John Terry, who he is aiming to rest on occasions this season, Gary Cahill and Branislav Ivanovic to play centre-half.

Kurt Zouma is close to getting match fit after a serious knee injury, but the shortage of numbers gave Michael Hector, who joined from Reading for £4m last August, an opportunity to be involved in pre-season.

In pictures: Chelsea vs Werder Bremen 9 show all In pictures: Chelsea vs Werder Bremen 1/9 Chelsea manager Antonio Conte barks out orders to his players. Chelsea FC via Getty Images 2/9 Eden Hazard dances past a challenge on the left flank Fabian Bimmer/Reuters 3/9 Chelsea midfielder Oscar looks for a pass after beating his man. Chelsea FC via Getty Images 4/9 New signing N'Golo Kante gets stuck in. Chelsea FC via Getty Images 5/9 Eden Hazard congratulates Diego Costa after the striker's goal. Chelsea FC via Getty Images 6/9 John Terry provides a composed presence in the centre of defence. Chelsea FC via Getty Images 7/9 Manager Antonio Conte urges his side to alter their shape. Chelsea FC via Getty Images 8/9 Substitute Ruben Loftus-Cheek bursts forward. Chelsea FC via Getty Images 9/9 Striker Diego Costa stretches to keep the ball in play. Chelsea FC via Getty Images 1/9 Chelsea manager Antonio Conte barks out orders to his players. Chelsea FC via Getty Images 2/9 Eden Hazard dances past a challenge on the left flank Fabian Bimmer/Reuters 3/9 Chelsea midfielder Oscar looks for a pass after beating his man. Chelsea FC via Getty Images 4/9 New signing N'Golo Kante gets stuck in. Chelsea FC via Getty Images 5/9 Eden Hazard congratulates Diego Costa after the striker's goal. Chelsea FC via Getty Images 6/9 John Terry provides a composed presence in the centre of defence. Chelsea FC via Getty Images 7/9 Manager Antonio Conte urges his side to alter their shape. Chelsea FC via Getty Images 8/9 Substitute Ruben Loftus-Cheek bursts forward. Chelsea FC via Getty Images 9/9 Striker Diego Costa stretches to keep the ball in play. Chelsea FC via Getty Images

Chelsea have earmarked other defenders to sign in case they fail to agree a deal for Koulibaly, including Benfica’s Victor Lindelof.

But with just over three weeks of the transfer window still to go, they will continue to focus on Koulibaly for now, with the suspicion it will take £50m to secure his signature.