"Technically Helmut is correct," Horner is quoted by international reports including De Telegraaf.
"Daniel's contract expires at the end of 2018 and it is the first time in his career that he is a free agent.
"But that doesn't mean that we don't want to keep him," Horner added.
"In the long term we want to continue with Max (Verstappen) and Daniel. The dynamics of Daniel's experience and Max's youthful enthusiasm works very well.
"And Daniel has had a very consistent season, scoring nine podiums," the Briton added.
"I am quite clear that our intention is to keep both drivers, I would say at least until 2020."
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