(CNN) -- The U.S. military 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.
Fierce fighting swept across the city Sunday after armed men stormed the country's interim Parliament. The violence appeared to be some of the worst since the 2011 revolution that ousted longtime leader Moammar Gadhafi.
Turmoil in Libya: Fighting sweeps across Tripoli following violence in Benghazi
Four additional V-22 Osprey aircraft "arrived overnight" at the naval base in Sigonella, Italy, to join four V-22s and 200 Marines that had been moved there last week, a U.S. defense source said Monday.
The V-22 Ospreys, which can take off and land vertically with at least two dozen passengers, are ready to be in the air on six hours notice, the official said. The additional aircraft should give the military the capability to evacuate more than 200 people from the embassy.
The aircraft and Marines are part of the Marine Air-Ground Task Force Crisis Response team, stationed in Moron, Spain. The force was formed after the attack on the U.S. diplomatic compound in Benghazi in 2012 to provide a closer standby military capability in a crisis.
At least two people were killed and 66 injured Sunday in Tripoli, according to the Health Ministry.
Fighters armed with heavy weapons moved in on the General National Congress as Sunday's session was adjourned. The attackers stormed the building as members were evacuated. Fighting then spread to other parts of the city.
The attack involved members of powerful militias from the western mountain city of Zintan.
The al-Qaaqaa brigade, a Zintan militia based in Tripoli, said in a statement that it had "heeded the call of the homeland to save it from the abusing politicians."
Two other militias from Zintan reportedly took part: al-Madani and al-Sawaeq.