Suruc, Turkey, Reuters—US-led air strikes hit Islamic State of Iraq and Syria (ISIS) positions around the Syrian border town of Kobani on Friday in an apparent bid to pave the way for heavily-armed Iraqi Kurdish Peshmerga forces to enter from neighboring Turkey.

The predominantly Kurdish town, besieged for more than 40 days, has become the focus of a global war against the Sunni Muslim insurgents, who have captured expanses of Iraq and Syria and declared an Islamic “caliphate” straddling the two.

Its fighters have killed or driven away Shi’ite Muslims, Christians and other communities who do not share their ultra-radical brand of Sunni Islam. They executed at least 220 Iraqi Sunnis in retaliation against opposition to their takeover of territory west of Baghdad this week.

The siege of Kobani, known in Arabic as Ayn al-Arab, has turned into a test of the US-led coalition’s ability to stop ISIS’s advance, with weeks of air strikes so far failing to break the insurgents’ stranglehold.

The arrival of the Iraqi Kurdish forces, known as Peshmerga, would be the first time Turkey has allowed ground troops from outside Syria to reach the border town to reinforce Syrian Kurds who have been defending it for weeks.

Machine gun fire could be heard from the town of Suruc on the Turkish side of the border, as ISIS fighters pounded the area near where the Peshmerga are expected to cross. Kobani’s defenders, outgunned by the militants, are hoping the Peshmerga, with badly-needed weapons including cannon and truck-mounted machine-guns, will help them turn the tide.

An advance guard of 10 Peshmerga briefly entered Kobani on Thursday to discuss a joint strategy with leaders of the YPG, the main Syrian Kurdish armed group defending the town.

Armored vehicles came and went from a former cotton processing warehouse near Suruc, where the wider contingent of around 150 Peshmerga fighters were preparing for action.

Tankers from the convoy emerged from the compound, guarded by Turkish security forces, to fill up at a local fuel station. Turkish President Tayyip Erdogan said the soldiers were awaiting instructions from Erbil, the capital of Iraq’s Kurdish region.

In Iraq, government forces and Kurds have made gains against ISIS in the north in recent weeks, but the US air strikes have failed to stop the militants from advancing in Anbar province, a vast western region straddling the Euphrates river valley from the Syrian border to Baghdad’s outskirts.

This week’s execution of more than 220 tribesmen who resisted ISIS’s advance in the Euphrates valley appears to be the worst mass killing of fellow Sunnis by a group previously known for massacring Shi’ites and non-Muslims.

At least 220 bodies of men from the Albu Nimr tribe, seized by ISIS days earlier, were found in mass graves in recent days. They had been shot at close range.

Many of Iraq’s Sunnis supported ISIS as it advanced through the country, seeing the fighters as protectors from the Shi’ite-led government in Baghdad.

Washington hopes that tribes can be persuaded to switch sides and help the Baghdad government fight against the militants, as they did in Anbar during the 2006-2007 “surge” campaign, the bloodiest phase of the US occupation of Iraq. But so far, tribes that resist ISIS have faced harsh retribution while complaining of scant support from Baghdad.

Iraq’s most senior Shi’ite cleric called on the government on Friday to rush to their aid.

“What is required from the Iraqi government . . .is to offer quick support to the sons of this tribe and other tribes that are fighting ISIS terrorists,” Grand Ayatollah Ali al-Sistani said, in an address read out by an aide in the holy city of Kerbala after Friday prayers.

“This will offer the opportunity to the other tribes to join the fighters against ISIS,” said the message from the reclusive 84-year-old cleric, whose pronouncements are seen by Shi’ites in Iraq and beyond as having the force of law.

Sheikh Naeem al-Ga’oud, a leader of the Albu Nimr, told Reuters he feared many more tribesmen would be rounded up, shot at close range and dumped in mass graves. He said his tribe had pleaded to the government for help in the days before its village fell to an ISIS onslaught.

“A day before the attack we told them (the government) that we will be targeted by the ISIS. I talked to the commander of the air force, with several commanders,” he told Reuters in an interview. “We gave them the coordinates of the places where they were, but nobody listened to us.”

The arrival of Iraqi Kurds through Turkey to help protect Kobani in Syria is a major political event in a conflict that has spread violence in the region.

Turkey has absorbed some 200,000 refugees from the Kobani area in recent weeks, but its failure to act to help protect the border town infuriated members of its own Kurdish minority, leading to riots in October in which around 40 people died.

Erdogan, who has been a reluctant supporter of the coalition but has allowed the passage of the Peshmerga from northern Iraq, said the US and its allies were too focused on Kobani and should also turn attention elsewhere.

“Why Kobani and not other towns like Idlib, Hama or Hom . . .while Iraqi territory is 40 percent controlled by ISIS?” Erdogan told a news conference in Paris after talks with President Francois Hollande. Erdogan said a peace process with Kurds in Turkey would continue despite the riots.

The US military said it continued to target ISIS militants near Kobani on Thursday and Friday. It said four air strikes damaged four fighting positions used by the militant group as well as one of its buildings.

“For the past 15 days, ISIS has been attacking to try to take control of the border gate, including with car bombs. But we are resisting,” said Enver Muslim, the top Kurdish administrative official in the Kobani district.

“While the Peshmerga convoy passes, US jets will be overhead and warplanes from the coalition . . .will be flying over Kobani to ensure their security,” he told Reuters by phone.

The Britain-based Syrian Observatory for Human Rights said on Friday preliminary information indicated that at least 21 ISIS members were killed in coalition air strikes around Kobani, including a Danish jihadist.

A local journalist in the town said there had been several air strikes overnight. A Reuters correspondent watching from across the border in Turkey saw one early on Friday to the east of Kobani.

Around 200 fighters from the Free Syrian Army (FSA), an umbrella term for dozens of armed groups fighting against both Syrian President Bashar Al-Assad and ISIS, have also entered Kobani from Turkey to help defend the town.

Turkey, which wants the US-led coalition’s aims in Syria to include Assad’s removal from power, has been a staunch supporter of the Syrian rebels, a policy which Hollande said France also endorsed.

“We are conscious that in Syria there are two enemies. That’s why we are . . .supporting the FSA, which was what we discussed, because we are sure that there will only be victory on the ground with the FSA,” he said.

White House spokesman Josh Earnest said US strategy against ISIS was working, denying Washington was focused on weakening the Islamic militants at the expense of its parallel goal of pushing Assad out of power.

The Peshmerga were given a heroes’ welcome as their convoy of jeeps and flatbed trucks crossed Turkey’s mainly Kurdish southeast this week, making their way towards Kobani from their base in northern Iraq’s Kurdistan region.

It is unclear whether the small but heavily armed contingent will be enough to swing the battle, but the deployment is a potent display of unity between Kurdish groups that often seek to undermine each other.

Assad’s government responded to the arrival of the Iraqi Peshmerga by condemning Turkey for allowing foreign fighters and “terrorists” to enter Syria in a violation of its sovereignty. Its foreign ministry described the move as a “disgraceful act”.

Turkey, which has made clear it will not send its own troops into Syria, dismissed the comments.