Chelsea are closing in on the €40 million signing of Napoli centre-back Kalidou Koulibaly while continuing to pursue an agreement with Manchester City for Nathan Ake, sources have told ESPN.

Thomas Tuchel's side are keen to strengthen their defensive options after Antonio Rudiger and Andreas Christensen left the club on free transfers earlier in the summer, joining Real Madrid and Barcelona respectively.

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Chelsea have so far been thwarted in their attempts to land Matthijs de Ligt from Juventus, with the Netherlands centre-back preferring a move to Bayern Munich.

Sources insist Chelsea have not given up hope on De Ligt but, for the time being, Chelsea are progressing with other targets and formal confirmation that Koulibaly has agreed to join from Napoli is expected in the coming days.

Personal terms are set to be finalised on what has been described by sources as a "long-term contract." Reports elsewhere have suggested Koulibaly, 31, has been offered a five-year deal.

As ESPN reported last week, talks are continuing between Chelsea and City over a deal for Ake, who is keen on a return to Stamford Bridge after coming through the youth ranks at the west London club.

City were holding out for £50m, but there is growing confidence at Chelsea that they can agree a deal for slightly less. Chelsea have asked to be kept informed of Bayern's talks with De Ligt. They also hold an interest in Paris Saint-Germain's Presnel Kimpembe.

Meanwhile, Raheem Sterling is flying to the United States to join his new Chelsea teammates for a formal unveiling after agreeing a £50m deal to leave City.