Diafra Sakho has told West Ham boss Slaven Bilic he wants to leave the club this summer, according to Sky sources.
Sky sources understand the forward spoke with Bilic over the weekend and asked to be sold ahead of next season.
Sunderland are keen on Sakho and tabled an £11m bid to sign him earlier this month.
The 26-year-old was signed by Sam Allardyce, now of Sunderland, from Metz in the summer of 2014 and was the top scorer for West Ham the following season with 12 goals in 26 games.
He was not as prolific last term, finishing with seven Premier League goals after missing time between November and March due to a thigh injury.
West Ham co-chairman David Sullivan told Sky Sports News HQ in May he was confident of securing a "top class forward" ahead of the club's move to the Olympic Stadium.
They have already had bids for Carlos Bacca, Alexandre Lacazette and Callum Wilson turned down, while Sky sources understand they remain interested in Michy Batshuayi and Christian Benteke.