US planes pounded Islamic State 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.
US 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.
British Prime Minister David Cameron recalled Parliament to vote on Friday on whether to join the air strikes. He said in an address at the UN that a comprehensive strategy was needed to combat Islamic State.
"Our strategy must work in tandem with Arab states, always in support of local people, in line with our legal obligations and as part of a plan that involves our aid, our diplomacy and, yes, our military," Cameron said at the UN.
"We need to act and we need to act now," he said.
Syrian Kurds said Islamic State 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 civil war.
Washington and its Arab allies killed scores of Islamic State fighters in the opening 24 hours of air strikes, 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 Kobani showed the difficulty Washington faces in defeating Islamist fighters in Syria, where it lacks strong military allies on the ground.
"Those air strikes are not important. We need soldiers on the ground," said Hamed, a refugee who fled into Turkey from the Islamic State advance.
Mazlum Bergaden, a teacher from Kobani who crossed the border on Wednesday with his family, said two of his brothers had been taken captive by Islamic State 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 Islamic State militants and Kurds could be seen from across the border in Turkey, where the sounds of sporadic artillery and gunfire echoed around the hills.
FRENCH HOSTAGE KILLED
Islamist militants in Algeria boasted in a video they had beheaded a French hostage captured on Sunday to punish Paris for joining air strikes against Islamic State in Iraq. French President Francois Hollande confirmed the execution.
"My determination is total and this aggression only strengthens it," Hollande said. "The military air strikes will continue as long as necessary."
The United States said it was still assessing whether Mohsin al-Fadhli, a senior figure in the al Qaeda-linked group Khorasan, had been killed in a US strike in Syria.
A US official earlier said Fadhli, an associate of al Qaeda founder Osama bin Laden, was thought to have been killed in the first day of strikes on Syria. The Pentagon said any confirmation could take time.
Washington describes Khorasan as a separate group from Islamic State, made up of al Qaeda veterans planning attacks on the West from a base in Syria.
As Obama tried in meetings in New York to widen his coalition, Belgium said it was likely to contribute warplanes in the coming days, and the Netherlands said it would deploy six F 16s to support US led strikes.
The initial days of US strikes suggest one aim is to hamper Islamic State's ability to operate across the Iraqi-Syrian frontier. On Wednesday US led forces hit at least 13 targets in and around Albu Kamal, 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 Qaim, the Iraqi town at the Albu Kamal border crossing. It also struck inside Iraq west of Baghdad and near the Iraqi Kurdish capital Arbil on Wednesday.
An Islamist fighter in the Albu Kamal area reached by phone said there had been at least nine strikes on Wednesday by "crusader forces". Targets included an industrial area.
US officials said air strikes under way in Syria late on Wednesday targeted oil infrastructure controlled by Islamic State, a move that appeared aimed at the militants' cash flow.
Perched on the main Euphrates valley highway, Albu Kamal controls the route from Islamic State's de facto capital Raqqa in Syria to the front lines in western Iraq and down the Euphrates to the western and southern outskirts of Baghdad.
Islamic State'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 battle there.
France, which has confined its air strikes to Iraq, said it would stay the course despite the killing of hostage Herve Gourdel, 55, a mountain guide captured on vacation in Algeria on Sunday by a group claiming loyalty to Islamic State.
In a video released by the Caliphate Soldiers group entitled "a message of blood to the French government", gunmen paraded Gourdel's severed head after making him kneel, pushing him on his side and holding him down.
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