By Jeff Mason and Tom Balmforth WASHINGTON/MOSCOW (Reuters) -The United States and its allies urged their citizens to leave Ukraine right away to avoid a Russian invasion, including a possible air assault, that Washington said on Friday could occur anytime. Moscow accused Western nations of spreading lies to distract from their own aggressive acts. The United States and Europe stepped up their warnings of an imminent attack while the Kremlin, jostling for more influence in post-Cold War Europe, rejected a joint EU-NATO diplomatic response to its demands to reduce tensions as disrespectful. Rus…