US President Donald Trump has said NATO countries should stop buying Russian oil if they want Washington to tighten sanctions on Moscow -- but achieving this could be time consuming and challenging. Only three NATO nations currently import Russian crude: Hungary, Slovakia, and Turkey. Of the three, Turkey is the big one. "According to our data, (Turkey) is the third largest Russian oil importer globally," Petras Kanitas, a Vilnius-based analyst at the Center for Research on Energy and Clean Air (CREA), told RFE/RL on September 15. "Turkey buys…