Damascus and Mürşitpınar, Reuters—US planes pounded Islamic State of Iraq and Syria (ISIS) positions in Syria for a second day on Wednesday, but the strikes did not halt the fighters’ advance in a Kurdish area where fleeing refugees told of villages burnt and captives beheaded.

President Barack Obama, speaking at the United Nations, asked the world to join together to fight the militants and vowed to keep up military pressure against them.

“The only language understood by killers like this is the language of force, so the United States of America will work with a broad coalition to dismantle this network of death,” Obama said in 40 minute speech to the UN General Assembly.

Islamist militants in Algeria claimed in a video they had executed a French hostage captured on Sunday to demand that France end its involvement in airstrikes against ISIS in Iraq, according the SITE monitoring service.

A US official said the leader of an Al-Qaeda unit called Khorasan had been killed in the first day of strikes on Syria. Washington describes Khorasan as a separate group from ISIS, made up of Al-Qaeda veterans planning attacks on the West from a base in Syria.

“We believe he is dead,” he said of Khorasan chief Mohsin Al-Fadhli, an associate of Al-Qaeda founder Osama Bin Laden.

Syrian Kurds said ISIS had responded to US attacks by intensifying its assault near the Turkish border in northern Syria, where 140,000 civilians have fled in recent days in the fastest exodus of the three-year conflict.

Washington and Arab allies have killed scores of ISIS fighters in the first 24 hours of airstrikes, the first direct US foray into Syria two weeks after Obama pledged to hit the group on both sides of the Iraq–Syria border.

However, the intensifying advance on the northern town of Ain Al-Arab was a reminder of the difficulty Washington faces in defeating ISIS in Syria, where it lacks strong military allies on the ground.

“Those airstrikes are not important. We need soldiers on the ground,” said Hamed, a refugee who fled into Turkey from the ISIS advance.

Mazlum Bergaden, a teacher from Ain Al-Arab who crossed the border on Wednesday with his family, said two of his brothers had been taken captive by ISIS fighters.

“The situation is very bad. After they kill people, they are burning the villages . . . When they capture any village, they behead one person to make everyone else afraid,” he said. “They are trying to eradicate our culture, purge our nation.”

Fighting between ISIS militants and Kurds could be seen from across the border in Turkey, where the sounds of sporadic artillery and gunfire echoed around the hills.

Focus on Iraqi border

The initial days of US strikes suggest one aim is to hamper ISIS’s ability to operate across the Iraqi–Syrian frontier. On Wednesday US-led forces hit at least 13 targets in and around Al-Bukamal, one of the main border crossings between Iraq and Syria, after striking 22 targets there on Tuesday, said the Syrian Observatory for Human Rights, a body which monitors the conflict in Syria.

The US military confirmed it had struck inside Syria northwest of Al-Qa’im, the Iraqi town at the Al-Bukamal border crossing. It also struck inside Iraq west of Baghdad and near the Iraqi Kurdish capital Erbil on Wednesday.

An Islamist fighter in the Al-Bukamal area reached by phone said there had been at least nine strikes on Wednesday by “crusader forces.” Targets included an industrial area.

Perched on the main Euphrates River valley highway, Al-Bukamal controls the route from ISIS’s de facto capital Raqqa in Syria to the frontlines in western Iraq and down the Euphrates to the western and southern outskirts of Baghdad.

ISIS’s ability to move fighters and weapons between Syria and Iraq has provided an important tactical advantage for the group in both countries: fighters sweeping in from Syria helped capture much of northern Iraq in June, and weapons they seized and sent back to Syria helped them in battles there.

France, which has confined its airstrikes to Iraq, where the government requested foreign intervention, said it would stay the course despite the killing of hostage Hervé Gourdel, a mountain guide captured on vacation in Algeria on Sunday by a group claiming loyalty to ISIS.

“We will not commit troops on the ground [but] we will stay as long as necessary until the Iraqi army has restored its superiority over [ISIS],” Prime Minister Manuel Valls told parliament in Paris.

Damascus: Campaign goes “in right direction”

The campaign has blurred the traditional lines of Middle East alliances, pitting a US coalition comprised of countries opposed to Syrian President Bashar Al-Assad against fighters that form the most powerful opposition to Assad on the ground.

The attacks have so far encountered no objection, and even signs of approval, from Assad’s Syrian government. Syrian state TV led its news broadcast with Wednesday’s airstrikes on the border with Iraq, saying “the USA and its partners” had launched raids against “the terrorist organization ISIS.”

US officials say they informed both Assad and his main ally Iran in advance of their intention to strike but did not coordinate with them.

Jordan, Bahrain, the United Arab Emirates, Qatar and Saudi Arabia have joined in the strikes. All are ruled by Sunni Muslims and are staunch opponents of Assad, a member of a Shi’ite-derived sect, and his main regional ally, Shi’ite Iran.

But some of Assad’s opponents fear the Syrian leader could exploit the US military campaign to rehabilitate himself in the eyes of Western countries, and that strikes against ISIS could solidify his grip on power.

In perhaps the strongest signal yet that Damascus wants to be seen as fighting the same battle as Washington, Syria’s minister for national reconciliation, Ali Haider, told Reuters: “What has happened so far is proceeding in the right direction in terms of informing the Syrian government and by not targeting Syrian military installations and not targeting civilians.”

Damascus was watching the events with caution, Haider said.

ISIS advances on Kurds

Even as ISIS outposts elsewhere have been struck, the fighters have accelerated their campaign to capture Ain Al-Arab, a Kurdish city on the border with Turkey. Nearly 140,000 Syrian Kurds have fled into Turkey since last week, the fastest exodus of the entire three-year conflict.

An ISIS source, speaking to Reuters via online messaging, said the group had taken several villages to the west of Ain Al-Arab. Footage posted on YouTube appeared to show ISIS fighters using weapons including artillery as they battled Kurdish forces near Ain Al-Arab. The Islamists were shown raising the group’s black flag after tearing down a Kurdish one.

A Turkish official, speaking on condition of anonymity, said any further advance by ISIS would trigger a fresh wave of refugees. The official said the advance had been very rapid three days ago but was slowed by the US-led airstrikes.

But Ocalan Iso, deputy leader of Kurdish forces defending Ain Al-Arab, said more ISIS fighters and tanks had arrived in the area since the coalition began airstrikes on the group.

“Ain Al-Arab is in danger,” he said, repeating calls for the coalition to expand its airstrikes to ISIS positions near the town. The Syrian Observatory reported airstrikes overnight near Ain Al-Arab, but the US military, Kurdish and Syrian officials did not confirm strikes in that area.

Iso said ISIS fighters had advanced to within 5 miles (8 kilometers) of the southern periphery of Ain Al-Arab—closer than they have been at any stage in the latest offensive.

Dozens of Syrian Kurds who had fled the fighting watched from a hillside on the Turkish side as Kurds battled ISIS militants in a cluster of villages called Siftek. The Kurds appeared to be firing mortars from the back of a truck into a village where ISIS had taken up positions.

“There are more and more ISIS fighters in the last two days, they have brought all their forces here,” said Ahmed Hassan, a 60-year-old Syrian Kurd who fled to Turkey with his family.

“They have heavy weapons. We are running away from them. The People’s Protection Units haven’t got heavy weapons, that’s why we need help,” he said referring to the main Kurdish armed group.

The campaign against ISIS marks the first direct involvement of the United States in Syria’s conflict, which started as a peaceful protest movement in 2011 but degenerated into an armed conflict after a government crackdown.

More than 190,000 people have been killed in the fighting and millions have fled their homes. Gun battles, bombings, shelling and airstrikes regularly kill over 150 people a day.