Carlos Sainz has admitted that staying at Toro Rosso for a fourth consecutive season in 2018 would be "difficult".
The Spaniard has in the past been linked with moves to Renault, Ferrari and most recently McLaren, but Red Bull are insisting his contract be honoured.
Sainz is quoted by El Mundo Deportivo as saying: "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. 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.
"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."
Sainz has picked up 25 points for the team so far in 2017, compared to teammate Daniil Kvyat's four.