Team boss Toto Wolff, who admits to having coffee with Red Bull driver Verstappen's father Jos at Monza, confirmed that Finn Bottas's newly-announced deal is for 2018 only.
"We are very happy with Valtteri and have therefore extended with him for 2018," Wolff told Bild.
"However, we still want to keep all our options open for 2019."
Verstappen has made clear his displeasure with Red Bull this season, but is believed to be firmly under contract not only for 2018 but also 2019.
However, Red Bull's driver manager Dr Helmut Marko said: "I cannot take offense if he looks around elsewhere."
The 19-year-old is sixth in the driver rankings heading into this weekend's Singapore Grand Prix.