Secretary of State Marco Rubio on Sunday said the U.S. will find out “pretty soon” if Moscow is serious about wanting peace in Ukraine, 24 hours ahead of President Donald Trump’s highly anticipated call with Russian President Vladimir Putin.

In an interview with CBS’s “Face the Nation,” Rubio said the U.S. is currently testing Russia’s position as it seeks to broker a ceasefire between the two countries.

“So I think your question is, are they tapping us along?” Rubio asked. “Well, that’s what we’re trying to find out. We’ll find out pretty soon.”

Rubio added that the U.S. expects to see some movement forward in the negotiations in order to stay engaged.

“We don’t want to be involved in this process of just endless talks. There has to be some progress,” Rubio said. “If, at the end of this, in the next few days, we get a document produced by both sides, and it shows that both sides are making concessions and being realistic and rational in their approach, then I think we can feel good about continuing to remain engaged.”