Carlos Sainz has admitted staying at Toro Rosso for a fourth consecutive season in 2018 would be "difficult".
The impressive Spaniard has in the past been linked with moves to Renault, Ferrari and most recently McLaren, but Red Bull is insisting his contract be honoured.
Sainz confirmed in Azerbaijan: "It’s no secret that my sporting career depends on Helmut Marko and Red Bull.
"But I do not think that will affect my future," he is quoted by El Mundo Deportivo. "I do not close the door to anyone but I know that a fourth year in Toro Rosso is extremely difficult because it’s never been done before.
"I also think both Red Bull and I want to take a step forward," Sainz added.
"It would be stupid to close the door to any F1 team, but I do not hide that my goal is to take a step, and if that step is Red Bull or elsewhere, I don’t know yet."