By Kinda Makieh and Jonny Hogg

DAMASCUS/MURSITPINAR Turkey (Reuters) - U.S. 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.

U.S. 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 U.N. 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 U.N. 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 U.N.

"We need to act and we need to act now," he said.

Syrian Kurds said Islamic State had responded to U.S. 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 U.S. 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.

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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 U.S. strike in Syria.

A U.S. 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 U.S.-led strikes.

The initial days of U.S. strikes suggest one aim is to hamper Islamic State's ability to operate across the Iraqi-Syrian frontier. On Wednesday U.S.-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 U.S. 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.

U.S. 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.

DAMASCUS: CAMPAIGN GOES "IN RIGHT DIRECTION"

The campaign has blurred the traditional lines of Middle East alliances, pitting a U.S. coalition comprising 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 air strikes on the border with Iraq, saying "the USA and its partners" had launched raids against "the terrorist organisation Islamic State."

U.S. 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 U.S. military campaign to rehabilitate himself in the eyes of Western countries, and that strikes against Islamic State could solidify his grip on power.

ISLAMIC STATE ADVANCES ON KURDS

Even as Islamic State outposts elsewhere have been struck, the fighters have accelerated their campaign to capture Kobani, 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 civil war.

An Islamic State source, speaking to Reuters via online messaging, said the group had taken several villages to the west of Kobani. Footage posted on YouTube appeared to show Islamic State fighters using weapons including artillery as they battled Kurdish forces near Kobani. 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 the advance had been rapid three days ago but was slowed by the U.S.-led air strikes.

But Ocalan Iso, deputy leader of Kurdish forces defending Kobani, said more militants and tanks had arrived in the area since the coalition began air strikes on the group.

"Kobani is in danger," he said.

More than 190,000 people have died in the Syrian conflict and millions have fled their homes. Gun battles, bombings, shelling and air strikes regularly kill over 150 people a day.

(Additional reporting by Arshad Mohammed, Steve Holland, Michelle Nichols and Louis Charbonneau at the United Nations, Phil Stewart in Washington, Patrick Markey in Tunis, Tom Perry, Sylvia Westall, Mariam Karouny, Laila Bassam, Alexander Dziadosz in Beirut, Suleiman al-Khalidi in Amman, Anthony Deutsch in The Hague; Writing by Peter Graff and Alexander Dziadosz; Editing by David Stamp, Chizu Nomiyama and Ken Wills)