WASHINGTON (Reuters) -The United States assesses that Russian strikes in western Ukraine in the past 24 hours were aimed at preventing airfields from being used by Ukrainian forces, a senior U.S. defense official said on Friday. The mayor of the city of Lutsk in western Ukraine, said four people were killed and six wounded in an attack on an airfield there, a rare strike on a target deep in western Ukraine far from the battlefields in the north, east and south. The official, who spoke on condition of anonymity, could not say the extent to which targeted airfields had been used by Ukraine’s air…