More now from Rubio, who says he doesn't expect a breakthrough in the conflict unless president Donald Trump is directly involved in the negotiations and engages with Russian President Vladimir Putin. He says this is important to determine what the Russian side wants and if peace is possible.

Russia and Ukraine agreed to exchange 1,000 prisoners of war each during peace talks held in Istanbul on Friday, in what would be the biggest prisoner swap of the conflict. The negotiations, which lasted less than two hours and were mediated by Turkey, marked the first time the nations have engaged in direct peace talks since the war began.