Navy ship with 1,000 Marines arrives off Libya to help Americans if unrest escalates

March 18, 2011: Then-Libyan senior Rebel commander Khalifa Hifter at a press conference in Benghazi, Libya. (AP)

The USS Bataan, with about 1,000 Marines aboard, has sailed into the Mediterranean Sea to assist in evacuating Americans if deadly fighting in Libya worsens, U.S. military officials said Tuesday.

Officials said the Navy amphibious assault ship sailed from the Arabian Sea and was already scheduled to go to the Mediterranean to participate in a multi-county military exercise in the region. However, they made clear the ship has received no formal orders to conduct new missions.

Also on Tuesday, the State Department called for all U.S. citizens to immediately leave Libya, saying the security situation in the country "remains unpredictable and unstable."

The fighting is taking place in the capital city of Tripoli, where renegade Libyan Gen. Khalifa Hifter is waging an offensive against Islamists.

Hifter began his so-called "Dignity Operation" more than 10 days ago to crush Islamist militias and their political backers.

The State Department said last week the U.S. embassy in Tripoli was open and operating under normal conditions amid the violence.

Hifter has the support of politicians, diplomats, army units and tribes that want him to impose order and rein in the country's unruly militias, three years after they toppled and killed longtime dictator Muammar al-Qaddafi.

However, the Al Qaeda-inspired group Ansar al-Shariah has now vowed to fight Hifter, whom it accuses of being an "American agent."

Ansar al-Shariah is believed to have played a role in the deadly Sept. 11, 2012, attack on the U.S. consulate in Benghazi, Libya, in which U.S. Ambassador Chris Stevens and three other Americans were killed.

The Associated Press contributed to this report.