The Hammers are keen to strengthen their squad as they fight for Premier League survival and McCormack is the latest target for manager Sam Allardyce, who has made a new striker a priority in January.
However, Leeds are standing firm on their position and have insisted that they will not let the 27-year-old leave Elland Road in the January transfer window.
"Leeds United have rejected an offer from West Ham United for striker Ross McCormack," the club said in a statement on their official website.
"The London club made an offer for the player on Friday afternoon which was rejected by the club's board."
McCormack moved to Leeds from Cardiff City in 2010.