US will know in 'matter of weeks' if Russia is serious about peace or using 'delay tactic': Rubio

"The Russians know our position in terms of wanting to end the war, and we will know from their answers very soon whether they are serious about proceeding with real peace or whether it is a delay tactic," Rubio said at NATO headquarters in Brussels. 

"[If] It's a delay tactic, the president's not interested in that," he added. "President Trump is not going to fall into the trap of endless negotiations about negotiations."

When pressed by reporters, Rubio wouldn't comment on what conditions Russia has set out in securing a peace deal. 

But the Kremlin later clarified that any agreement was contingent on the West lifting sanctions.

"I guess it's part of the game," Rubio said. "At the end of the day, what's going to matter here is whether we're going to move towards peace or not."

Rubio reiterated that Ukraine and Russia would both need to make concessions to end the war but declined to say what those should be, insisting those details should emerge through negotiations.

"Initially, it was important to talk [to the Russians] because we haven't talked to them in a long time. But now we've reached the stage [where] we need to make progress," he said, noting it will be "hard," but he remains "optimistic."

"There are some promising signs. There are some troubling signs. It's not going to be easy. No one ever said this would be easy, but we're going to find out sooner rather than later," Rubio told reporters. "And let's just say I'm hopeful. I remain hopeful."