WASHINGTON (Reuters) - U.S. Defense Secretary Chuck Hagel ordered an aircraft carrier moved into the Gulf on Saturday, readying it in case Washington decides to pursue a military option after insurgents overwhelmed a string of Iraqi cities this week and threatened Baghdad.

"The order will provide the Commander-in-Chief additional flexibility should military options be required to protect American lives, citizens and interests in Iraq," the Pentagon said in a statement.

The carrier USS George H.W. Bush, moving from the North Arabian Sea, will be accompanied by the guided-missile cruiser USS Philippine Sea and the guided-missile destroyer USS Truxtun, the statement said. It added the ships were expected to complete their transit into the Gulf later on Saturday.

President Barack Obama said on Friday he needed several days to determine how the United States would help Iraq deal with the stunning advance of Islamist militants, who earlier this week seized several major Iraqi cities and appeared to have set their sights on the capital, Baghdad.

But the U.S. leader ruled out sending U.S. troops back into combat in Iraq, where over 4,000 U.S. soldiers died in the war that followed the 2003 invasion to oust Saddam Hussein, and said any intervention would be contingent on Iraqi leaders overcoming their longstanding political and sectarian divisions.

While the United States has already increased its surveillance assistance to Iraq, conducting drone flights at the request of the government of Iraqi Prime Minister Nuri al-Maliki, the Pentagon has also prepared a range of options for Obama to consider, including conducting air strikes.

Any stepped-up U.S. actions would be aimed at helping Iraq counter militants of the Islamic State in Iraq and the Levant, or ISIL, who are seeking to establish an Islamic caliphate across much of Iraq and Syria.[ID:nL2N0OU0XR]

The USS George H.W. Bush is a Nimitz-class aircraft carrier, the largest warships in the world, according to the U.S. Navy. They are powered by two nuclear reactors and can carry a crew of about 6,000.

Story continues

In addition to fighter jets, helicopters and other aircraft, the ships are equipped with sophisticated anti-ship and anti-aircraft missiles.

One U.S. defense official declined to say how the USS H.W. Bush might be used to help Iraq fend off ISIL fighters, but said that such vessels are often used to launch airstrikes, conduct surveillance flights, do search, rescue, humanitarian and evacuation missions, and conduct seaborne security operations.

"Carriers can do all this from the sea, without needing any other country's permission," the official said on condition of anonymity.

(Reporting by Frances Kerry and Missy Ryan; Editing by Eric Beech and Eric Walsh)