A small band of ISIS fighters mounted a bold, dawn attack Friday on an air base in Iraq where U.S. forces are training troops, in what experts believe may have been a probe that could foreshadow a coming clash involving Americans.

All eight Islamic State fighters were killed by Iraqi forces in the 7:20 a.m. raid on Ain al-Asad air base in the western province of Anbar, and none of the 400 U.S. military personnel at the base were involved, according to officials.  Sources rejected reports that the militant fighters, who killed at least two Iraqi troops, had infiltrated the base wearing Iraqi army uniforms.

"It was a regular ISIL attack,” a senior defense official told Fox News. “It happened at the gate about 3 kilometers from where U.S. forces are.  We sent drones and Apaches [attack helicopters], but it was all over before they arrived and neither fired a shot."

"They were signaling that our advisers were vulnerable, thus -- they hope -- driving Obama to pull them out"

- Retired US Army Lt. Col. Ralph Peters

Army Col. Steve Warren, a Pentagon spokesman, said that initial reports Friday showed that all of the Islamic State fighters were killed at or inside a gate to the sprawling, 25-square-mile base.

Islamic State clearly never intended to take over the massive base with a team of eight fighters, said retired U.S. Army Lt. Col. Ralph Peters, who liked the attack to the so-called "Doolittle Raid" during World War II, in which the U.S. hit Tokyo and other locations with airstrikes following Pearl Harbor, sending the message Japan was vulnerable to attack and giving the enemy a hint of what was coming.

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"At al-Asad, the terrorists were doing three things," said Peters, a Fox News contributor. "They were signaling that our advisers were vulnerable, thus -- they hope -- driving Obama to pull them out; demonstrating to jihadi-world that they're standing up to the Americans, and replying to Obama's claim that Islamic State has been stopped and turned back."

The base has been the target of sporadic mortar fire in past weeks, and lies just a few miles from al-Baghdadi, a town on the Euphrates River ISIS reportedly overran in recent days. The jihadist army has been moving forces from its strongholds in Syria to Anbar Province, possibly setting the stage for a major clash with forces on the base that is now the sole bulwark between ISIS and Baghdad.

Militants took al-Baghdadi, seizing the police station and several government buildings, after invading from all sides amid a sandstorm.

"Ninety percent of al-Baghdadi district has fallen under the control of the insurgents," district manager Naji Arak told Reuters.

But Defense Department spokeswoman confirmed to Fox News that “heavy fighting” took place in al-Baghdadi on Thursday, and on Friday, sources said that Iraqi troops were making progress in clearing ISIS fighters.

There are currently nearly 2,600 U.S. forces in Iraq, including about 450 who are training Iraqi troops at three bases across the country, including al-Asad. Forces from other coalition countries conduct the training at the fourth site, in the northern city of Irbil.

But even if Islamic State militants close in on the base, taking it would require a massive force, that would present a target for airstrikes, retired Col. Thomas Lynch, a National Defense University fellow, told Fox News.