(Adds number of military personnel, Kerry calls, State Department condemnation)
By Missy Ryan and Jim Loney
WASHINGTON, June 15 (Reuters) - The United States ordered military personnel to boost security for its diplomatic staff in Baghdad on Sunday and said some staff were being evacuated from the embassy as the Iraqi government battled to hold off insurgent forces.
"A small number of DOD (Department of Defense) personnel are augmenting State Department security assets in Baghdad to help ensure the safety of our facilities," Pentagon spokesman John Kirby said in a statement. A U.S. military official said fewer than 100 people would be involved, including marines and other soldiers.
Separately, the State Department said it was evacuating some staff from the embassy and beefing up security at the sprawling facility.
"Some additional U.S. government security personnel will be added to the staff in Baghdad; other staff will be temporarily relocated - both to our Consulate Generals in Basra and Arbil and to the Iraq Support Unit in Amman," the State Department said in a statement, without giving numbers.
The Baghdad embassy was reviewing staffing requirements but a "substantial majority" of the embassy presence in Iraq would remain in place, it said.
The Pentagon statement said that for the moment embassy staff would be evacuated using commercial, charter and State Department aircraft. But the U.S. military had airlift assets at the ready should they be needed.
The State Department also advised other U.S. citizens in Iraq to exercise caution and to limit travel to five provinces including restive Anbar in the west and Kirkuk in the north. U.S. citizens were also advised to register with a State Department program called Smart Traveler. The program enables U.S. embassies to stay in touch with citizens abroad in an emergency.
The U.S. government moves came as Iraqi government forces battled to hold off insurgents with the Islamic State in Iraq and the Levant, or ISIL, the Sunni militant group that has seized control of parts of northern Iraq.
After sweeping through towns north of the capital, the ISIL militants' advance appeared to have halted outside Baghdad as fighters moved instead to tighten their grip on the north.
President Barack Obama said on Friday he needed several days to determine how the United States would help Iraq deal with ISIL's stunning advance. But he ruled out sending U.S. troops back into combat in Iraq, which U.S. troops departed in 2011 after a bloody, costly war that was launched by his predecessor, George W. Bush, and lasted more than eight years.
Secretary of State John Kerry discussed the ISIL threat in Iraq and in neighboring Syria with senior officials from Jordan, Saudi Arabia, Qatar and the United Arab Emirates, a senior State Department official said.
HEAVILY FORTIFIED EMBASSY
The heavily fortified U.S. embassy in Baghdad occupies a sprawling swathe of land along the Tigris River, inside the secure Green Zone, where many Iraqi government buildings are located and which is off-limits to most Iraqis.
When the embassy opened in 2009, it included thousands of diplomats, servicemen, and officials from 14 U.S. government agencies. It also has a vast warren of office buildings, apartment blocs, a swimming pool and a grocery store.
Dubbed the 'mega-bunker of Baghdad,' its size reflected the scale of the U.S. investment in Iraq following the 2003 invasion to oust Saddam Hussein, and an expectation the United States would have some sort of major long-term presence there.
The embassy's staff was later trimmed back in line with the U.S. military transition out of Iraq, which U.S. troops departed in late 2011.
U.S. officials have declined to say how many staff work there currently. But according to a 2013 State Department inspector general report, the embassy was moving to reduce its headcount from over 11,500 in January 2013 to 5,500 in January 2014.
In 2013, the United States also had diplomatic outposts in Arbil, Basra, and the contested city of Kirkuk.
The security of U.S. diplomats has been a charged political issue for the Obama administration since the Sept. 11, 2012 attack on a diplomatic facility in Benghazi, Libya, which resulted in the death of Ambassador Chris Stevens and three other Americans.
Speaking to reporters last week, Pentagon spokesman John Kirby said that U.S. Central Command, which is responsible for Iraq, had "assets and resources" and existing plans should the United States need to evacuate personnel.
"But we're not there yet," he told reporters on Friday.
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.
The State Department also condemned on Sunday reports that ISIL had massacred 1,700 Shi'ites in the Iraqi city of Tikrit.
"While we cannot confirm these reports, one of the primary goals of ISIL is to set fear into the hearts of all Iraqis and drive sectarian division among its people," State Department spokewoman Jen Psaki said in a statement.
"The United States will do its part to help Iraq move beyond this crisis and we urge all Iraqis to unite against this violence and continue to reject the path of hatred that ISIL represents," she said. (Reporting by Jim Loney and Missy Ryan; Editing by Peter Cooney and Frances Kerry)