Iraqi forces outside the town of Akashat in Anbar province on Sept. 15 prepare for military operations against the Islamic State. (Moadh Al-Dulaimi/AFP/Getty Images)

Iraqi forces backed by U.S. airstrikes began an assault on the Islamic State in western Anbar province Tuesday, breaching one of the last two militant strongholds in Iraq where the group's elusive leader, Abu Bakr al-Baghdadi, is thought to be possibly hiding.

A force composed of army units, police and tribal fighters from the area launched the attack at dawn near the town of Ana, located on the Euphrates River about 60 miles from the Syrian border, Iraq's military said in a statement.

Additional troops from Iraq's elite counterterrorism forces are expected to join the fight as it moves west toward the border with Syria.

The battle for the remaining Islamic State bastions in Anbar is expected to be complex because of the porous Syrian border and the vast desert terrain, which is difficult to surround and choke off. The challenges in Anbar are well known to U.S. forces after years of combat against al-Qaeda in the province a decade ago.

U.S. intelligence officials believe that 5,000 to 10,000 militants are in the area, moving easily between Anbar and the neighboring Syrian province of Deir al-Zour, which they still largely control.

Iraqi and U.S. military officials have said Baghdadi is probably holed up in the region, moving between safe houses along the border.

There have been frequent claims that Baghdadi was killed in an airstrike, but the reports have not been corroborated. Last month, the outgoing commander of U.S. coalition forces, Lt. Gen. Stephen Townsend, said he believes that Baghdadi is still alive, contradicting Russian assertions that the militant leader probably died in an airstrike.

Tuesday's announcement came without the usual fanfare that has accompanied such campaigns in the past, underscoring how far Iraq has come in diminishing the Islamic State's influence and territorial dominance in the country.

The Islamic State has been evicted from 90 percent of the Iraqi cities and towns it held, including the northern city of Mosul, and the launch of each battle was usually accompanied by a televised speech from Prime Minister Haider al-Abadi.

The Anbar campaign is expected to push westward from Ana along the Euphrates to the town of Rawah and end in the border outpost of Qaim.

Brett McGurk, the White House envoy for the campaign against the Islamic State, said in a Twitter post early Tuesday that "major operations" were underway in western Anbar. A spokesman for the U.S.-led coalition confirmed in a separate Twitter post that the Iraqi forces were being backed by American airstrikes.

The Islamic State is now under pressure from three large offensives in one of its last major territorial holdings, the Euphrates River Valley, which straddles Iraq and Syria. From the west, Syrian regime forces backed by Russia and Iran are moving on Deir al-Zour. Meanwhile, U.S.-backed forces are pushing into the province from the north. With the start of operations in Iraq on Tuesday, the Islamic State is also being pressed from the east.

The convergence of these forces, which often have competing interests and loyalties, sets the stage for a complicated military campaign that puts rival forces fighting a common enemy into proximity, raising the possibility of clashes.

Iraqi forces are separately preparing to fight for the northern town of Hawijah, a battle that has been delayed and complicated by a political dispute over who will control it once the Islamic State is evicted.

Hawijah sits in Kirkuk province, which is due to participate Monday in a controversial referendum on Kurdish independence from Iraq. Kurds and Arabs both have a historical claim to the province and have jockeyed for position over who should lead the fight for Hawijah.