Iraqi security forces rest on a sidewalk following clashes with jihadists on Sept. 19 in Ramadi, the provincial capital of the western province of Anbar and the birthplace of al-Qaeda in Iraq. (Azhar Shallal/AFP/Getty Images)

Islamic State militants are threatening to overrun a key province in western Iraq in what would be a major victory for the jihadists and an embarrassing setback for the U.S.-led coalition targeting the group.

A win for the Islamic State in Anbar province would give the militants control of one of the country’s most important dams and several large army installations, potentially adding to their abundant stockpile of weapons. It would also allow them to establish a supply line from Syria almost to Baghdad and give them a valuable position from which to launch attacks on the Iraqi capital.

The Islamic State’s offensive in Anbar has received less attention than its assault on the Syrian border city of Kobane, which has played out in view of news photographers standing on hills in nearby Turkey. But in recent weeks, Islamic State fighters have systematically invaded towns and villages in Anbar, besieged army posts and police stations, and mounted attacks on Iraqi troops in Ramadi, the provincial capital.

The Islamic State secured a major foothold in Anbar province in January when it seized the city of Fallujah and parts of Ramadi. It pushed farther into the province in June, but Iraq’s government was able to maintain small pockets of authority in the majority-Sunni region.

Iraqi forces have suffered numerous reverses­ in the latest jihadist offensive, including the loss of two army bases. U.S. warplanes and attack helicopters have hit Islamic State targets and provided support to Iraqi troops fighting in Anbar. The U.S. airstrikes helped fend off an assault last month on the Haditha Dam, part of the militants’ drive to control Iraq’s water supplies. But overall, the strikes have failed to curb the militants’ momentum.

“If the Islamic State controls Anbar, they would be able to threaten serious targets in Baghdad,” said an Iraqi security expert, Saeed al-Jayashi. “The government would lose the Haditha Dam, and the security forces would have to retreat,” he said. “There would be a blood bath.”

Anbar province — Iraq’s largest — was the epicenter of the Sunni insurgency against U.S. forces­ that raged after the invasion in 2003. In 2006, Anbar’s numerous Sunni tribes decided to back the U.S.-supported government against Iraq’s al-Qaeda affiliate, in what later became known as the Sunni Awakening. The insurgency was crushed.

But in recent years, the sectarian policies of former prime minister Nouri al-Maliki, a Shiite, alienated the Sunni tribes and their constituencies. The Islamic State, which had been founded as an offshoot of al-Qaeda in Iraq, fed off the Sunni discontent. At the same time, the jihadists improved their military prowess by fighting in the civil war in Syria. They have seized large chunks of Syria and Iraq.

Since the beginning of the campaign against the Islamic State in August, U.S. warplanes and helicopters have struck more than 40 targets in Anbar province, according to U.S. Central Command data.

The Obama administration had expressed hope that Sunni Arab powers in the region, led by Saudi Arabia, would persuade the Anbar tribes to turn against the Islamic State and join Iraqi government forces­ or participate in a locally based national guard.

But although Maliki left office early last month, there has been little indication that Arab influence, if indeed it is being used, has had much of an effect. At the same time, Sunni tribesmen have said they feel threatened by the Shiite militias that are participating in Iraq’s fight against the Islamic State.

In talks this week with retired U.S. Gen. John Allen, the administration’s coordinator of the international coalition against the Islamic State, tribal leaders said that “they will not confront the Islamic State while Shiite militias exist in Sunni areas,” tribal chief Samil al-Muhammadi told the Saudi-owned London newspaper Al-Hayat.

Psychological meaning

Anbar province, a vast expanse of desert crisscrossed by truck routes leading to Jordan, Saudi Arabia and Syria, holds both strategic and symbolic significance for the Islamic State.

If the extremist group captures the territory, it could funnel weapons and fighters from areas it controls in Syria all the way to the western outskirts of Baghdad. Currently, that supply line is interrupted by government-held Haditha and Ramadi.

The militants would also extend their de facto border to just outside the Iraqi capital.

“It will be a base for their movements. It would take a very long time to get it back,” said Anbar’s police chief, Ahmed Saddak al-Dulaimi.

The capture of Anbar would also be a psychological victory for the jihadists.

Anbar “is really the birthplace of ISIS’s predecessor organization, al-Qaeda in Iraq,” said Jessica D. Lewis, research director at the Institute for the Study of War in Washington, using a common name for the Islamic State. “So taking the cities of Anbar province is quite important to ISIS.”

Security officials in Anbar say the Islamic State has been bolstering its fighting force in the province.

In the past few days, the militants have wrested control of the Anbar town of Hit on the Euphrates River, as well as the nearby town of Kubaisa. Both are close to the Ayn al-Asad military base, one of Iraq’s largest. It sends reinforcements and supplies to troops defending the Haditha Dam just northwest of the camp.

According to a recent assessment by the Institute for the Study of War, the Islamic State has conducted a “sophisticated campaign” in Anbar in the past four weeks, which has enabled the group to control most of the territory from the Syrian border to Abu Ghraib in the western suburbs of Baghdad.

The militants have severed the Iraqi army’s supply lines, cut off troops’ communications and consolidated gains that would not be easily disrupted by an air campaign, the report said.

Perhaps most alarming is the jihadists’ advance on Ramadi, 80 miles west of Baghdad.

Iraqi news media outlets reported Monday that security forces­ had withdrawn from central Ramadi, a claim that Dulaimi, the police chief, later denied. But attacks over the past week have left the militants in control of new neighborhoods in the city.

Local officials have warned the central government that Ramadi may soon fall.

“All of the areas around Ramadi are controlled by the Islamic State,” said Ahmed Abu Risha, a prominent tribal sheik who commands pro-government fighters in the area.

Abu Risha said his forces­, who are lightly armed, have received no air support while fighting off the Islamic State.

“If Ramadi falls, all of Anbar falls,” he said. “Ramadi is the head. If you cut the head, the rest of the body will die, too.”

One of the most important losses­ for the Iraqi security forces­ was the military camp at Saqlawiyah. Islamic State fighters surrounded the base west of Fallujah last month. Some of the soldiers there fled, while the jihadists are believed to have massacred many others, according to survivors. Between 300 and 500 soldiers were missing, they said. The militants subsequently seized a military base at Albu Aytha, 50 miles from Baghdad.

“For days we begged for airstrikes and they never came,” said a 38-year-old soldier who survived the onslaught at Saqlawiyah and gave his name only as Abu Ali for fear of retribution.

Now, he says, he doesn’t believe there is anything worth fighting for in Anbar.

“The leadership doesn’t care about us, the people there [in Anbar] don’t care about us. They called us Shia dogs,” he said. “How can I fight for any of them after this?”

Jayashi, the analyst, said that Anbar residents needed to support the Iraqi security forces­.

“Otherwise,” he said, “we will lose all of western Iraq.”

Karen DeYoung in Washington and Mustafa Salim in Baghdad contributed to this report.