Daniel Ricciardo is not ruling out leaving Red Bull to drive for another Formula 1 team.

Red Bull officials have repeatedly claimed that the Australian and his teammate Max Verstappen have "watertight" contracts for 2018, but Ricciardo expressed some frustration that the team are not in a title-challenging position so far this year.

"Obviously now it gets to that point where we really - and I - want to be fighting for a championship," he told Fairfax Media.

"It can't kind of be a long-term plan anymore to me.

"Obviously I thought this year would be a chance and that seems to be slipping away. So we have got to see some real progress over the year and a sign that next year it can be delivered, because it is just, obviously, personal goals."

Asked directly about the prospect of leaving Red Bull, however, Ricciardo said: "At the moment, no.

"I don't really know what they are going to do, to be honest. It is all pretty much a closed book I guess at this stage. So yeah, I would say it is still an unlikely for next year."

The 2017 calendar continues with the Monaco Grand Prix tomorrow.