Sheikh Naim al-Gaoud, a Sunni Muslim leader of the Albu Nimr tribe, called for more U.S. intervention -- including ground troops, arming tribes directly or at least pressuring the Iraqi government to give the tribes more firepower.
While U.S. officials have said that ISIS, which calls itself the Islamic State, is on the defensive in Iraq and Syria, al-Gaoud says that's definitely not the case where he is.
"In Anbar, we are losing ground, not gaining," he said.
Thousands of families had been under siege in the town of Jubbat al-Shamiya until getting help Friday from U.S.-led coalition airstrikes and Iraqi forces, according to al-Gaoud.
But he said Iraqi troops had pulled out of Jubbat al-Shamiya on Saturday, at which time ISIS was shelling the town.
If the Islamist extremist group's fighters go in, al-Gaoud predicted a massacre.
Key base attacked
Anbar province is just west of Baghdad, meaning a decisive ISIS victory would put militants on the footsteps of the Iraqi capital. It's home to the strategic Ayn al-Assad Air Base, which came under attack Friday
Talking about that battle, Pentagon spokesman Rear Adm. John Kirby said 20 to 25 people -- most, if not all, of whom were wearing Iraqi military uniforms and were led by suicide bombers -- attacked the nearly 25-square-mile base.
"It looks like (ISIS militants) at least got to the outer base limits," Kirby said.
At least 13 Iraqi soldiers died in the assault, said al-Gaoud, which ended with Iraqi ground forces killing all the attackers.
U.S. troops were on the base at the time, but "several kilometers" from where the fighting happened, Kirby said. The U.S. military did deploy attack helicopters in that ISIS assault, but the Apaches returning safely without firing a shot, military sources said.
American helicopter gunships were also involved in a fight supporting Iraqi ground forces about 15 kilometers (9 miles) north in the Anbar town of al-Baghdadi, according to sources.
Al-Gaoud, the Albu Mimr tribal leader, said militants killed at least 25 Iraqi police officers during their assault on that town Thursday and Friday.
On Saturday, the U.S. military said al-Baghdadi was "contested," as Iraqi forces fought back.
Anbar province key in multiple ways
Anbar is important not just for its location, for the al-Assad base or for the Haditha dam, Iraq's second largest. It's significant for its sectarian breakdown -- as a mostly Sunni province in a Shiite-led country.
Sectarian divisions have hurt Iraq before, with ISIS' rampage through much of Iraq (as well as neighboring Syria) blamed in part to the country's lack of unity. It's one reason for then-Prime Minister Nuri al-Maliki's exit last year, replaced by current leader Haider al-Abadi.
The U.S. government has gotten involved to address such tensions as part of its anti-terrorism fight, such as President Barack Obama's warning last June
-- a few months before al-Maliki stepped down -- that "there won't be a military solution" unless Shiites, Sunnis and Kurds all play significant parts in Iraq's command structure.
Years before, in the mid-2000s, the United States recruited and paid Sunnis like members of al-Gaoud's Albu Mimr tribe to join its fight against al Qaeda. Those efforts helped turn the tide in the war.
But now, al-Gaoud says, ISIS -- which consists of Sunni extremists -- is making his tribe pay the price.
"There are people who will be killed in cold blood, and there will be more massacres," al-Gaoud told CNN in November. "We are getting killed because of our friendship with the Americans. Does a friend abandon his friend like this?"