Ukrainian President Petro Poroshenko expressed cautious hope Thursday that a peace settlement with Russia would stand and at least temporarily freeze the fighting along the country’s Eastern border.

“I have careful optimism,” Poroshenko said of the emerging plan while speaking to reporters before a meeting with Secretary of State John Kerry at the NATO Summit in Wales.

Ukrainians, he said, are “fighting for democracy, fighting for freedom” and are “mobilized to defend the sovereignty and territorial integrity of their country.”

“But at the end of the day, all of us want peace,” he added.

Earlier Thursday, Poroshenko attended a meeting with President Obama and the leaders of Britain, France, Germany and Italy.

Russian President Vladimir Putin on Wednesday announced that he and Poroshenko had agreed to the outlines of a peace plan but did not elaborate on the details. He said only that the Ukrainian army must draw back from the areas held by pro-Russian separatists and both sides would allow international monitors to step in and make sure the cease-fire holds.

In his public comments Thursday, Poroshenko specified only two broad demands: that Russia withdraw its troops from conflicted regions in Ukraine and stop sending more soldiers, arms and supplies across the border.

Putin and Poroshenko said more detailed peace talks would begin Friday in Minsk, the capital of Belarus, but both sides have agreed to observe a cease-fire starting Friday at 2 p.m. Ukraine time.

“I want to thank the United States for the strong and continued support of the peace effort,” he said, adding that “we cross our fingers” that Friday’s talks will cement a peace plan.

While Poroshenko was meeting with allies in Wales, the Kremlin on Thursday issued a stern warning to Poroshenko not to join NATO, threatening that any move to do so would disrupt efforts to end the fighting in eastern Ukraine.

Neither Poroshenko nor Kerry addressed the threat in their public remarks.

Kerry said Obama, and other Western leaders and allies, had a “frank discussion” about the challenges in Ukraine.

“President Poroshenko was very clear about his desire to find a peaceful way forward, but in the absence of that, the need for strong action by those people who support Ukraine,” he said.

Obama, he said, is “very committed” to continue moving on the latest round of sanctions against Russia while waiting to see if a ceasefire holds and Russia follows through with promises to leave Ukraine and respect its border.