U.S. President Donald Trump stated today that within two weeks he will know whether his Russian counterpart, Vladimir Putin, is “stalling” and warned that he will change course if it becomes clear that Russia is not serious about achieving peace in Ukraine.

Trump declined to specify what actions he might consider to penalize Russia, but hinted that potential new sanctions on Moscow could jeopardize the peace agreement. He emphasized that his recent sharp rhetoric toward Moscow “should speak for itself,” CNN reports.

“Words speak quite loudly—we are not happy with the current situation,” Trump said.

When asked whether he believes Putin wants to end the war, the U.S. president responded that he would be able to answer that more clearly in “about two weeks.”

“We’ll find out if he’s stalling or not—and if he is, we’ll respond a little differently,” Trump stated.

He added that he still believes Russia “seems to want to do something,” but also suggested uncertainty about how serious Moscow really is about ending the war. He stressed the need to be cautious when it comes to imposing new sanctions during ongoing peace efforts.

“I think I’m close to securing an agreement, and I don’t want to ruin it by introducing sanctions,” Trump said.