Everton have rejected a second Chelsea bid of £26 million for John Stones as they battle to keep the England defender.
The Premier League champions made the offer on Tuesday, sources have told ESPN FC.
It came after an initial offer of £20 million was turned down by the Toffees on July 17.
Neither Everton nor Chelsea have made any comment on the latest offer.
But Roberto Martinez said after the first bid was rejected that the 21-year-old centre-back was not for sale.
Martinez sees Stones, who has won four England caps already, as a future Everton captain.
He is looking to strengthen in central defence, having allowed Antolin Alcaraz and Sylvain Distin to leave when their contracts expired at the end of June.
Martinez is interested in Manchester United's Jonny Evans and has been linked with Dynamo Kiev defender Aleksandar Dragovic.
But he still sees Stones and club captain Phil Jagielka as his first-choice central defensive pairing for the coming season.
Stones was brought to Goodison Park by then-Everton boss David Moyes from Barnsley for £3 million in January 2013, and is under contract until 2019.
The defender, who can play at right-back as well as in the centre, said during Everton's recent preseason trip to Singapore that he was happy to stay at the club.