June 16, 2014 -- About 275 Marines and Army troops were deployed to the U.S. Embassy in Baghdad today and four warships steamed towards Iraq as tensions rose due to the advance of a radical Islamic insurgency.

The U.S. took the additional precautions as ISIS -- the Islamic State of Iraq and Syria -- expanded its territory in Iraq by capturing the city of Tal Afar, which is west of Mosul. In addition, fresh details emerged of a massacre carried out by ISIS, including top Muslim clerics who refused to pledge loyalty to the fanatic Islamic group.

Defense Secretary Chuck Hagel has ordered the aircraft carrier USS George H.W. Bush and two other ships from the northern Arabian Sea and it has arrived in the Persian Gulf as President Obama considers possible military options for Iraq. A fourth ship, the amphibious ship USS Mesa Verde, was ordered into the Persian Gulf today.

The Mesa Verde carries approximately 550 Marines and about five Osprey planes, tilt rotor craft that can take off and land like helicopters, but fly like planes.

Pentagon spokesman Rear Adm. John Kirby said deploying the USS Bush will give Obama additional flexibility if military action were required to protect American citizens and interests in Iraq.

The Obama administration is also considering talks with its long time nemesis Iran to help with political reconciliation in Iraq. They would take the form of informal discussions on the sidelines of the Iran denuclearization talks currently taking place in Vienna.

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The threat of a massive ISIS attack on Baghdad has prompted the U.S. Embassy to evacuate a significant number of employees to other U.S. diplomatic facilities in Iraq and bring in the specially-trained Marines, as well as 50 additional Army personnel, State Department officials said.

U.S. travelers in the country were encouraged to exercise caution and limit travel to certain parts of Iraq.

"Due to the relocation of personnel from Baghdad, the embassy will only be restricted in its ability to offer all consular services; but emergency services are always available to U.S. citizens in need at any embassy or consulate anywhere in the world," State Department spokeswoman Jen Psaki said.

Iraq has yet to mount a concerted effort to confront ISIS, a group so radical that al Qaeda disowned it, and roll back the effect of its lightning strikes last week through much of northern Iraq. It is now poised to strike at Baghdad.

Thousands of young Iraqi men have volunteered to fight the militants, including some barely in their teens. A marching band was there for Monday's sendoff as the young men were loaded into open trucks.

The new recruits will face a battle-hardened army that doesn't flinch at atrocities. The jihadist group released graphic images this weekend showing mass executions of Iraqi soldiers. The group is threatening to impose strict Islamic law, killing whoever opposes it.

Fresh reports of massacres emerged today from Mosul and other areas Under ISIS' control.

UN High Commissioner for Human Rights Navi Pillay said "it appears that hundreds of non-combatant men were summarily executed over the past five days, including surrendered or captured soldiers, military conscripts, police and others associated with the Government."

The victims included the imam of the Grand Mosque in Mosul when the imam refused to pledge allegiance to ISIS. Twelve other imams were also executed in front of Al Israa mosque, Pillay said.

ISIS has claimed to have executed 1,700 conscripts after they took over the city of Tikrit.

Iraqi forces have been struck back with helicopters launching rockets at targets near the main road leading to Baghdad. The city was rocked by a string of explosions, with as many as 15 dead, Iraqi police and hospital officials said.

Top Republicans, such as South Carolina Sen. Lindsey Graham, say action is needed.

"If Baghdad falls, if the central government falls, a disaster awaits us of monumental proportions," Graham said Sunday on "State of the Union."

The Associated Press contributed to this report.I