Displaced people from the minority Yazidi sect, fleeing the violence in the Iraqi town of Sinjar, walk to a refugee camp in August. Islamic State militants seized two villages on Oct. 20 and are closing in on Mount Sinjar. (Youssef Boudlal/Reuters)

Islamic State militants advanced on Mount Sinjar on Monday, seizing two villages and blocking roads as besieged fighters from the minority Yazidi sect pleaded for U.S.-led airstrikes to save them.

Yazidi volunteers who have been protecting the area for more than two months said they retreated from the villages north of the mountain after the extremists attacked in the early hours of Monday under the cover of bad weather. The Yazidis pulled back to a shrine in the foothills of the mountain but said the militants were closing in — their armored vehicles visible just a few miles away as night fell.

“We have so little ammunition, and they are advancing,” said Khalid Qassim Shesho, a 44-year-old fighter trapped in the Sharfadin shrine. “I can see five Humvees without using binoculars. We need planes!”

The extremist gains around Mount Sinjar strike an embarrassing blow to the international campaign against the Islamic State. In August, President Obama authorized targeted airstrikes in Iraq to address the plight of thousands of Yazidis trapped on Sinjar in the face of an initial militant onslaught.

However, the airstrikes have shown a limited ability to turn the tide against the extremists. In the western province of Anbar, the Sunni militants have seized territory and army bases, and they have the provincial capital in their sights. Overrunning Anbar would give the Islamic State a valuable perch from which to increasingly target Baghdad and Shiite areas to the south, which were shaken by multiple bombings Monday.

The Islamic State says that because Yazidis are polytheists, they deserve brutal treatment. Yazidi men have been slaughtered and hundreds of women captured, with the Islamic State recently admitting to enslaving them and buying and selling them as concubines.

The numbers of those stranded Monday was unclear. Tens of thousands of Yazidis were displaced from the area during the Islamic State’s August assault, but some remained in more-remote parts of the mountain and foothill villages that had not been seized.

Only about 20 families were left in Dohula, the first village to come under attack Monday, and they fled at the first sign of an assault, Yazidi fighters said. Dohula is also known by its Arabic name, al-Qadisiyah.

Yazidi gunmen were then forced to retreat to Bork, where a few families had already fled, before pulling back to the Sharfadin shrine, which has become a makeshift military outpost. Bork is also known as Yarmouk.

Attacking from the north, the Islamic State has blocked roads that had served as an evacuation route for Yazidis two months ago when they were stranded on Sinjar with little food, water or shelter .

“We called asking for help with airstrikes, but nothing,” said Mohammad Khalil, a Yazidi and former member of parliament who has been fighting alongside the 2,000-strong volunteer force in the area since late August. “Now we are all trapped — the fighters are trapped, the civilians are trapped.”

Vian Dakhil, a Yazidi lawmaker in Baghdad, said that as many as 2,000 people were trapped on the mountain Monday.

“If Daesh reaches them, they will kill them all,” she said, using the Arabic acronym for the extremist group.

Haji Ghandour, another Yazidi member of parliament, said that although those forced to flee their villages Monday were largely fighters, about 700 Yazidi families remain in the area, many encamped on Sinjar.

“If the situation doesn’t change, we’ll see an event worse than in August,” he said.

Islamic State militants used mortars and rockets in Monday’s attack, killing “dozens,” according to an online statement purported to be from the extremist group. “Clashes are ongoing until this time and progress is continuing,” the statement said, adding that the group had destroyed the Sharfadin shrine, a claim that Yazidi fighters there said was untrue.

Ali Qassem, a 26-year-old from Bork, said he was at the shrine sheltering with 400 other volunteer fighters who were being targeted by mortar fire.

Kurdish forces also were attempting to help repel the Islamic State’s advances from an artillery position on the mountain, he said.

As night fell and the Islamic State’s Humvees crept closer, about 30 Kurdish soldiers arrived at the shrine from their encampment atop the mountain to provide reinforcements. They brought heavier weapons than the aged AK-47s that the volunteers carry, but fighters still feared that they may not be able to hold out.

“It’s very hard, and we are worried about how long we can hold,” said Faris Badl, a 40-year-old Yazidi. “But we can’t retreat. We need to protect the families on the mountain.”