Red Bull official Marko told Diario Sport: "The rumours are wrong. Red Bull is the one who decides about drivers."
For his part, Sainz is not jumping the gun.
"At the moment I'm still a Renault driver," he told Auto Motor und Sport.
"If Red Bull asks me to come back, that would of course change. I owe Red Bull a lot. The mutual respect is great.
"I also have a very good relationship with Helmut Marko. It's thanks to him that I'm in F1."
The 2018 calendar continues with the Belgian Grand Prix on August 26.