MOSCOW, Feb 13 (Reuters) - The United States has told its citizens to leave Russia immediately due to the war in Ukraine and the risk of arbitrary arrest or harassment by Russian law enforcement agencies."U.S. citizens residing or travelling in Russia should depart immediately," the U.S. embassy in Moscow said. "Exercise increased caution due to the risk of wrongful detentions.""Do not travel to Russia," it added."Russian security services have arrested U.S. citizens on spurious charges, singled out U.S. citizens in Russia for detention and harassment, denied them fair and transparent treatment, and convicted them in secret trials or without presenting...