The U.S. military has doubled the number of aircraft standing by in Italy if needed to evacuate Americans from the U.S. Embassy in Tripoli, Libya, CNN has learned. A decision to evacuate as violence in the Libyan capital grows is "minute by minute, hour by hour," a defense official told CNN on Monday. Fierce fighting swept across the city Sunday after armed men stormed the country's interim Parliament. Sporadic bursts of gunfire and blasts could still be heard on the outskirts of the capital Monday evening. The violence appeared to be some of the worst since the 2011 revolution that ousted longtime leader Moammar Gadhafi.