The Netherlands forward has two years remaining on his contract with the Bundesliga outfit, and insists that he will not decide his future until he has spoken to his club.
"Regarding the rumours about Liverpool it would only be fair to speak to Schalke first," he told De Telegraaf.
"I've talked with Schalke about an extension for a while. After the World Cup I am having a holiday. Then I will speak to them again.
"With my agent, I now do not talk about the interest from clubs. I focus on the tournament."
The 30-year-old has struck 87 goals in his four-year spell with the German club.