The base outside the city of Hit was one of the Shiite-led government's few remaining military outposts in Anbar, a predominantly Sunni province that has increasingly fallen under ISIS control.
It is a key control point for roads running through the region.
The Iraqi military still controls the Ayn al-Asad military base, which helps defend Iraq's second-largest dam and the provincial capital of Ramadi.
Despite airstrikes by the U.S. and its allies over the weekend, reports suggest ISIS has continued to gain ground and has encircled Haditha, the last large town in Anbar not yet in the militant group's hands.
Sabah Al-Karhout, president of the Anbar Provincial Council, said Sunday that ISIS, the self-proclaimed Islamic State, had control of about 80% of the province.
Should all of Anbar fall, the Sunni extremists would rule from the perimeter of Iraq's capital to Raqqa in Syria, at least, according to the provincial council's deputy head, Falleh al-Issawi. ISIS would control a swath 350 miles (563 kilometers) long.
Anbar leaders on Saturday appealed for U.S. ground forces to help them fight ISIS, but Baghdad said it had not received any official request.
The Iraqi government has been adamant that it does not want U.S. forces on the ground, and President Barack Obama has not shown any intent to deploy any.
Meanwhile in Syria, fighting continues in the key Kurdish border city of Kobani, which ISIS has been trying to seize for weeks.
Monday has been one most violent days since ISIS launched its assault, with sounds of fierce fighting, including gunfire and explosions, CNN staff on the Syria-Turkey border said.
CNN's Nick Paton Walsh described seeing a mushroom cloud rising about 100 meters (nearly 330 feet) above the city in an area targeted by at least four blasts, generally after the sound of jets overhead.
"However, it remains unclear who is gaining the upper hand," Walsh said. "Distribution of the airstrikes does not immediately suggest the Kurds are retaking the center so far."
The U.S. military said it and partner nations had attacked ISIS in Syria on Sunday and Monday, launching four airstrikes southwest of Kobani, three northeast of the city and one northwest of Raqqa.
"Fighter aircraft from the Kingdom of Saudi Arabia participated in these airstrikes. All aircraft exited the strike areas safely," the U.S. military said in a statement.
A fighter from the Kurdish People's Protection Unit, or YPG, told CNN's Arwa Damon that the battle was focusing on the main border crossing into Turkey. If ISIS took control, he said, "it's over."
The fighter said the Kurdish fighters had pushed back an attempted advance by ISIS on Monday morning but that it would be "impossible" for them to hold their ground if current conditions continued.
U.S. national security adviser Susan Rice said Sunday that Turkey had agreed to allow the United States and its partners to use bases and territory to train.
"They have said that their facilities inside Turkey can be used by coalition forces, American and otherwise, to engage in activities inside Iraq and Syria," Rice told NBC's "Meet the Press."
"That's a new commitment and one that we very much welcome," she said.