By Khaled Yacoub Oweis and Dominic Evans AMMAN/BEIRUT (Reuters) - U.S. President Barack Obama voiced concern at Hezbollah's growing role in the Syrian civil war on Monday after men from the Lebanese militia fought their biggest battle yet alongside President Bashar al-Assad's army. About 30 Hezbollah fighters were killed on Sunday, Syrian activists said, along with 20 Syrian soldiers and militiamen loyal to Assad during the fiercest fighting this year in the rebel stronghold of Qusair, close to the Lebanese border. ...