The Red Devils are understood to be chasing the Holland international, with new boss Louis van Gaal a big fan of the Feyenoord ace.
Janmaat is almost certain to leave Rotterdam this summer after entering the final 12 months of his contract, with a cut-price switch on the cards.
United were hopeful that the situation may allow them to snap the 24-year-old up for just £4m, but the Eredivisie giants have told them to up their offer.
With Newcastle also loitering, Feyenoord believe that they can get £7m for the defender, and are keen for United to move closer to their valuation.
Janmaat has proven himself to be able on the international stage after starring for van Gaal’s Netherlands side, who are potentially just 90 minutes away from the World Cup Final.
United are also watching the full-back’s club an international team-mates Stefan de Vrij and Bruno Martins Indi.