Dr Helmut Marko has admitted he is "worried" Max Verstappen may leave Red Bull.

"Of course I am," the Red Bull driver manager told Italy's La Gazzetta dello Sport.

He is also worried Pirelli is supplying what he describes as "Mercedes tyres" in 2019.

"At the moment there is only one car that is winning and it is Mercedes," said Marko.

"We have taken a step backwards even though our performance is better," he added.

Verstappen, undoubtedly one of the hottest properties on the driver market, is under contract until the end of 2020 but Marko has admitted the deal includes a performance clause.

"This year's Red Bull is not as good a car as we had last year," Verstappen told Ziggo Sport in Montreal.

And he says the problem isn't the improving Honda engine.

"I think Honda is doing well, but of course it can always be better," said the Dutchman.

"Above all, we must continue to work as a team on improving the car," the 21-year-old said.

Toyoharu Tanabe, the new Honda boss in F1, said the Japanese manufacturer is happy to be working with Verstappen.

"For now we are very happy with this collaboration," he told the Dutch newspaper Algemeen Dagblad.

"The communication with Max is very nice and all about improving the engine. His feedback is very simple, precise and clear. That makes it much easier for us to work on a problem.

"He only says something if he considers it necessary, but that's ok for us. As long as he asks the necessary questions and points out problems. And he does," Tanabe added.

He said he doubts Verstappen will eventually turn on Honda, in the way that Red Bull's former relationship with Renault broke down.

"I don't think we can expect anything emotional from Max, as long as we remain productive," said the Japanese.

As for when Red Bull-Honda will finally close the gap to Mercedes, though, Tanabe will not name a precise date.

"It remains difficult to put a timeframe on it," he said.

"Mercedes is of course not standing still either. Of course we see the gap, so we must continue to push.

"Our package is ok now -- not fantastic, but not disappointing either."

And so the goal for both Red Bull and Honda is to give Verstappen a winning car as soon as possible, especially with contract negotiations set to begin.

"For the remainder of the year, Max must be confident that he has the best package here. We aim for Max to take another win this year," said Honda's Tanabe.

"He knows how to get the most out of his material, and that is very nice for an engine supplier to work with."