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 Monday evening.

The violence appeared to be some of the worst since the 2011 revolution that ousted longtime leader Moammar Gadhafi.

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In a move that could further inflame an already tense situation, the speaker of the interim parliament, Nuri Abu Sahmain, who is backed by Islamist forces, ordered troops known as the "Central Libya Shield Forces" to deploy to the capital Monday, the Libyan state news agency LANA reported.

The forces, mostly from the city of Misrata, east of Tripoli, are considered to be among the most powerful Islamist-affiliated militias. They have had long-running rivalries with the heavily armed Zintan militias when both groups were based in the capital.

Meanwhile, the Saudi ambassador to Libya announced that his country's embassy and consulate in Tripoli closed Monday because of the violence, and the staff has left Tripoli, according to the official Saudi Press Agency. The sites will reopen when the situation stabilizes, Ambassador Mohammed Mahmoud Al-Ali said, according to the report.

Turkey took similar measures, shutting down its consulate in Benghazi, Turkey's semi-official Anadolu news agency reported.

U.S. aircraft arrive in Italy

Four additional U.S. 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.

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 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. Fighting then spread to other parts of the city.

Libya's main political forces have been slowly dividing along Islamist and liberal lines.

The more liberal parties, backed by the heavily armed militias from the western mountain city of Zintan, have accused the Islamists of hijacking power and controlling the government and parliament.

The GNC attack involved the al-Qaaqaa brigade, a Zintan militia based in Tripoli, which said in a statement that it had "heeded the call of the homeland to save it from the abusing politicians."

Libya's political process has stalled as a result of infighting among the Islamist and liberal forces in the GNC, and elections for a new parliament to replace it have not yet taken place. Many Libyans view the GNC as having lost legitimacy.