Manchester United are interested in signing Issa Diop and willing to pay £45m plus a player for the West Ham defender, Sky Sports News understands.
West Ham are adamant that Diop is not for sale and it is believed they would not even listen to offers of less than £60m.
Diop played 38 times last season after the Hammers signed him from Toulouse a year ago for £22m.
United's top priority is signing a centre-back this summer and they have been told by Napoli that Kalidou Koulibaly would cost about £100m while Leicester value Harry Maguire at £80m.
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Former United manager Jose Mourinho singled out Diop for praise when his side lost 3-1 at West Ham in September last year.
As well as a centre-half, United are intent on bringing a new right-back to Old Trafford to replace departing club captain Antonio Valencia and are preparing a second bid for Crystal Palace's Aaron Wan-Bissaka after their first £40m offer was rejected.
Wales international winger Daniel James has already arrived at the club from Swansea City for £15m (plus £3m add ons).
Meanwhile, West Ham are intent on adding to their squad this summer and saw a £29m Maxi Gomez bid accepted by Celta Vigo on Saturday.
They have also launched a £13m bid for Eibar defensive midfielder Joan Jordan.
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