The Daily Record says United have made an improved Â£25million offer to sign Bellingham.
The Red Devils offered a hefty downpayment with add-ons expected to take the bid over the Â£12.5m-mark for the 16-year-old.
But the transfer fee could be double this amount.
Bellingham has emerged as an important first team player for Birmingham this season, despite his young age.
He fits the bill of what Ole Gunnar Solskjaer is looking for at United in that he has the best years of his career ahead of him and he's homegrown.