Three days after signing Luke Beckett, one of the Football League's most sought-after talents, from Stockport County, Neil Warnock's side last night made an official approach for the 22-year-old centre-forward.
And I can reveal that United's board have also sanctioned offers for TWO other strikers currently attached to top-flight clubs, although Leeds' Brett Ormerod and Matt Jansen from Blackburn are not among them.
Mellor, who scored both goals during Liverpool's recent Carling Cup victory over Middlesbrough, is understood to be growing increasingly disillusioned at the lack of first team opportunities at Anfield.
West Brom are also pursuing the youngster and United's decision to confirm an interest in two other, as yet unnamed, targets is an insurance policy to guard against being gazumped in the market.
United scouts watched Mellor, who scored against them in the Worthington Cup two seasons ago, in action for Liverpool's reserves on Monday night and, according to sources inside Bramall Lane, the initial responses from Merseyside have been encouraging.
Likewise the employers of the other players who feature on Warnock's most wanted list, although United plan a major charm offensive to try and persuade them to do business sooner rather than later.
Morocco international Hassan Kachloul is training with United following his departure from Aston Villa last season.
The midfielder has also played for Wolverhampton Wanderers, Southampton and St Etienne.