US fighter jets and bombers have pounded Islamic State targets, striking the militants in both Syria and Iraq.
The new wave of attacks came even as the extremists pressed their offensive in Kurdish areas within sight of the Turkish border, where fleeing refugees told of civilians beheaded and towns torched.
President Barack Obama, speaking at the United Nations in New York, vowed an extended assault and called on the world to join in.
"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," he told the UN General Assembly in a 38-minute speech. "Today, I ask the world to join in this effort."
In Syria, hardline rebels aligned with a faction fighting to oust President Bashar Assad but considered too radical by the US packed up their heavy weapons and fled their bases over fears the Obama administration would target all fighters deemed a potential threat to the United States.
Today's strikes marked the second day of a broadened US military operation against the Islamic State (IS) group, after a barrage of more than 200 strikes on some two dozen targets in Syria a day earlier.
That campaign, which the White House has warned could last years, builds upon the air raids the US has already been conducting for more than a month against the extremists in Iraq.
The ultimate aim of the Obama administration and its Arab partners is to destroy IS, which through brute force has carved out a proto-state in the heart of the Middle East, effectively erasing the border between Iraq and Syria. Along the way, the extremist faction has massacred captured soldiers, terrorised religious minorities and beheaded two American journalists and a British aid worker.
Today Algerian extremists aligned with IS declared in a video that they had beheaded a fourth hostage - a Frenchman seized in Algeria on Sunday - in retaliation for France joining the aerial assault against the militants in Iraq. President Francois Hollande said France would not be deterred by the act of "barbarity".
"This particular group, they don't strike only those who don't think like they do. They also strike Muslims. They rape, they kill," a visibly upset Mr Hollande told the UN. "It is for this reason that the fight the international community needs to wage versus terrorism knows no borders."
Meanwhile, US allies lined up in support of the aerial campaign. The Dutch government announced it would send six F-16 fighter jets along with 250 pilots and support staff to strike at Islamic State targets in Iraq, while Parliament has been recalled to debate Britain's response to a request to support the air strikes.
The latest US strikes damaged eight IS vehicles in Syria near the Iraqi border town of Qaim, the US Central Command said. It also reported hitting two IS armed vehicles west of Baghdad, as well as two militant fighting positions in northern Iraq.
Pentagon spokesman Rear Admiral John Kirby said the strikes in eastern Syria hit a staging area used by the militants to move equipment across the border into Iraq.
He did not specify exactly where the air raids took place, but the Iraqi town of Qaim is across the border from the Syrian town of Boukamal, where Syrian activists reported at least 13 air strikes on suspected IS positions today.
He later said US and coalition forces hit 12 targets, including oil refineries that were providing up to two million dollars (£1.2m) a day in income to IS.
Despite the start of the coalition campaign, IS fighters pressed their advance against Syrian Kurdish militiamen around the town of Ayn Arab, known to Kurds as Kobani, near the Turkish border, where refugees fleeing into Turkey reported the beheading of captives and the torching of homes.
A Kurdish militiaman fighting to protect the city said militants were less than half a mile from the outskirts.
Weary refugees arriving in Turkey described atrocities at the hands of the militants. Osman Nawaf, 59, said he saw about 50 bodies hanging headless in the village of Boras when he passed it on his three-day walk from a village on the outskirts of Kobani.
Halil Aslan, 48, a villager in Turkey, recounted seeing IS tanks roll into a village on the Syrian side.
"They shelled the place with tanks and mortars," he said. "We could hear them falling on those hills."
A video posted online showed what appeared to be IS fighters toting assault rifles and fanning out across a dusty field in the Kobani area. A later clip showed a field cannon firing a shell towards a town located across a rolling expanse of brown fields, followed by a puff of smoke in the distance.
For Syria's opposition, there is a cruel irony in the fact that, just over one year after the cancellation of planned missiles strikes to punish Assad's chemical weapon use, American and Arab warplanes and missiles are finally in Syria's skies.