Max Verstappen says Red Bull will need to up their game in Formula 1 over the next 18 months if he is not going to be tempted to leave.

Reliability problems have frustrated the Dutchman this year, leaving him sixth in the drivers' standings with just 67 points after 11 races.

He remains under contract with Red Bull until the end of 2019, but has warned the Milton Keynes-based outfit that things could change.

Speaking at last weekend's Zandvoort DTM race, Verstappen said his commitment to Red Bull in the long-term depends on how competitive the car they provide for him is.

"I've always said that there's no reason to leave when I have a competitive car," said Verstappen.

"At the moment we are not there where I want to be, but that doesn't mean you have to give up straight away.

"You have to work hard.

"But after two, three years without improving, that would be a different story.

"We are working hard to improve but after next year we will see what happens."

Red Bull team boss Christian Horner is confident Verstappen won't be tempted to join Ferrari or Mercedes.