Suruc, Turkey and Baghdad, Reuters—A convoy of Iraqi Kurdish forces in Turkey rolled late on Friday across the border into Syria to help Syrian Kurds defend the besieged town of Kobani that has become the focus of a Western-backed war against Islamic State of Iraq and Syria (ISIS) insurgents.

US-led air strikes hit ISIS positions around Kobani earlier in the day in an apparent effort to pave the way for the heavily-armed Kurdish contingent to enter.

The Iraqi Kurdish fighters, known as Peshmerga or “those who defy death”, had set off cheering and making victory signs in more than a dozen trucks and jeeps, accompanied by armored vehicles and artillery. They headed from a holding point around 8 km (5 miles) from the frontier towards Kobani.

“We have crossed over,” one of the Peshmerga fighters in the group subsequently told Reuters by telephone.

The force numbers only around 150 but brings weapons and ammunition. Their arrival would mark the first time Turkey has allowed ground troops from outside Syria to reinforce Syrian Kurds, who have been defending Kobani for more than 40 days.

As the Peshmerga headed towards the border, a loud blast was heard in the Kobani area, the latest in a rapid series of explosions, in an apparent intensification of the fighting.

Despite having limited strategic significance, Kobani has become a powerful international symbol in the battle against the hardline Sunni Muslim insurgents who have captured large expanses of Iraq and Syria and declared an Islamic “caliphate”.

The Kobani battle has raged in full view of the Turkish frontier, testing whether a US-led coalition can halt ISIS’s advance. The failure of Turkey to help defend the town sparked riots among Turkish Kurds in which 40 people died.

ISIS militants have killed or displaced Shi’ite Muslims, Christians and other communities deemed enemies of their ultra-radical brand of Sunni Islam. They executed at least 220 Iraqi Sunnis in retaliation for opposition to their takeover of territory west of Baghdad this week.

Earlier on Friday, machine-gun fire could be heard from the Turkish side of the border as ISIS fighters pounded the area near where the Peshmerga were expected to cross.

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 insurgents from advancing in Anbar, a vast western desert province straddling the Euphrates river valley from the Syrian border to Baghdad’s outskirts.

This week’s execution of tribesmen who resisted ISIS’s advance in the Euphrates basin appears to be the worst mass killing of fellow Sunnis by a group previously known for slaughtering 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. They had been shot at close range.

Many Iraqi Sunnis supported ISIS as it advanced through the north and west of the country in the first half of the year, seeing the fighters as protectors from the Shi’ite-led government in Baghdad.

With a new government under a Shi’ite prime minister seen as more conciliatory having taken office in September, Washington hopes that tribes can be coaxed to switch sides and help fight the militants, as they did in Anbar during the 2006-07 “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 Karbala after Friday prayers.

“This will offer the opportunity to the other tribes to join the fighters against Daesh,” 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 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 US State Department said it was deeply concerned by reports of the mass executions. ISIS’s “indiscriminate crimes prove, yet again, that it is targeting all Iraqis, regardless of faith or religion,” it said.

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 across 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 US-led coalition but has allowed the passage of the Peshmerga from northern Iraq, said Washington and its allies were too focused on Kobani and should also turn attention elsewhere.

“Why Kobani and not other towns like Idlib, Hama or Homs (in Syria) … 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 U.S. 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, U.S. 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 fighters were killed in coalition air strikes around Kobani, including a Danish jihadist.

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.

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.