Story highlights Turkey joins Saudi Arabia in shutting down diplomatic missions in Libya

Islamist militia troops ordered into Tripoli by speaker of parliament

At least four people were killed and 90 injured in violence Sunday

Eight V-22 Osprey aircraft and 200 U.S. Marines are on standby to evacuate Americans

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 Mondaywith 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 closer standby military capability in a crisis.

4 killed, dozens more injured in violence

At least four people were killed and 90 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.