The Iraqi army is under serious pressure in the west of the country, officials said, with the world's attention fixed instead on the Syrian town of Kobane as it battles a similar Islamic State in Iraq and the Levant offensive.
"It's tenuous there. They are being resupplied and they're holding their own, but it's tough and challenging," a senior US defense official, who spoke on condition of anonymity, told the AFP news agency.
Dozens of US-led air strikes in recent weeks on the western Anbar province, including near the city of Ramadi, have helped counter the group's fighters while the capital Baghdad remained secure, the official added.
ISIL has also so far failed to take the strategic Haditha dam, with US-led coalition bombing raids helping the Iraqi government fend off assaults.
The group seized the city of Fallujah in the Anbar province in January, and have repeatedly attempted to fully take Ramadi and the Haditha dam.
But the situation has illustrated how the Iraqi troops are far from an effective force and in urgent need of training, officials said.
The difficult circumstances in Anbar offered a stark contrast with battlefield reports from the country's north, where more capable Kurdish soldiers have made some headway.
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"There's no comparison" between the ability of the Kurdish forces and the Iraqi government army, the official said.
"The Kurds are moving, they're taking back towns and territory," and were able to coordinate with coalition forces, the official added.
A second US defense official told AFP that conditions in western Iraq were a cause for concern.
"It's not a good situation," he said.
Anbar province was the main battleground for a Sunni insurgency against US troops that erupted after the US-led invasion of 2003.
Sunni tribes later sided with the Baghdad government and American forces against al-Qaeda's branch in Iraq, in an offensive that US military officers dubbed the "Anbar awakening".
But the Shia-led government of former Prime Minister Nouri al-Maliki angered and alienated Anbar's Sunni community, and ISIL has sought to exploit the deep resentment felt by some in Iraq's west.