By Arshad Mohammed and Tom Perry

NEW YORK/BEIRUT (Reuters) - French fighter jets struck Islamic State targets in Iraq on Thursday, and the United States hit them in Syria, as a U.S.-led coalition to fight the militants gained momentum with an announcement that Britain would join.

The French strikes were a prompt answer to the beheading of a French tourist in Algeria by militants, who said the killing was punishment for Paris' decision last week to become the first European country to join the U.S.-led bombing campaign.

In the United States, FBI Director James Comey said Washington had identified the masked Islamic State militant in videos with a knife at the beheading of two American hostages in recent weeks. Those acts helped galvanize Washington's bombing campaign.

"I'm not going to tell you who I believe it is," Comey told reporters. He said he knew the person's nationality, but declined to give further details.

A European government source familiar with the investigation said the accent indicated the man was from London and likely from a community of immigrants. U.S. and European officials said the principal investigative work identifying the man was conducted by British government agencies.

Iraqi Prime Minister Haidar al-Abadi, in New York to attend a U.N. meeting, said on Thursday he had credible intelligence that Islamic State networks in Iraq were plotting to attack U.S. and French subway trains.

Senior U.S. officials and French security services said they had no evidence of the specific threat cited by Abadi. But New York Police Commissioner William Bratton said the department boosted its presence on subways and city streets after the Iraqi warning.

City officials added there was no specific, credible threat, and Mayor Bill de Blasio said: "We are convinced New Yorkers are safe."

A spokesman for Washington's metro said there had been no credible threats made against the U.S. capital's rail and bus system.

A senior Iraqi official, speaking on condition of anonymity, said: "A full assessment of the veracity of the intelligence and how far the plans have gone into implementation is ongoing.

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"We cannot further discuss the nature of the threat in the media," said the official, "except to reaffirm that Daesh (Islamic State) will continue to endanger international peace and security unless it is eradicated."

France had said earlier on Thursday it would boost security on transport and in public places after the killing of French tourist Herve Gourdel by Islamic State sympathizers in Algeria.

Britain, the closest U.S. ally in the past decade's wars, announced on Thursday that it too would join air strikes against Islamic State targets in Iraq, after weeks of weighing its options. Prime Minister David Cameron recalled parliament, which is expected to give its approval on Friday.

While Arab countries have joined the coalition, Washington's traditional Western allies had been slow to answer the call from U.S. President Barack Obama. But since Monday, Australia, Belgium and the Netherlands have said they would send planes.

The Western allies have so far agreed to join air strikes only in Iraq, where the government has asked for help, and not in Syria, where strikes are being carried out without formal permission from President Bashar al-Assad. France said on Thursday it did not rule out extending strikes to Syria, too.

Overnight, U.S.-led air strikes in eastern Syria killed 14 Islamic State fighters, according to a monitoring group, while on the ground, Kurdish forces were reported to have pushed back an advance by the Islamists toward the border town of Kobani.

The air raids follow growing alarm in Western and Arab capitals after Islamic State, a Sunni militant group, swept through a swath of Iraq in June, proclaimed a "caliphate" ruling over all Muslims, slaughtered prisoners and ordered Shi'ites and non-Muslims to convert or die.

'HARSHNESS, BRUTALITY, TORTURE AND MURDER'

More than 120 Islamic scholars from around the world, including many of the most senior figures in Sunni Islam, issued an open letter denouncing Islamic State. Challenging the group with theological arguments, they described its interpretation of the faith as "a great wrong and an offense to Islam, to Muslims and to the entire world."

"You have misinterpreted Islam into a religion of harshness, brutality, torture and murder," said the letter, signed by figures from across the Muslim world from Indonesia to Morocco.

A third night of air raids by the United States and Arab allies targeted Islamic State-controlled oil refineries in three remote locations in eastern Syria to try to cut off a major source of revenue for the al Qaeda offshoot.

The strikes also seem to be intended to hamper Islamic State's ability to operate across the Syria-Iraq frontier.

Obama has vowed to keep up military pressure against the group, which advanced through Kurdish areas of northern Iraq this week despite the air strikes. Some 140,000 refugees have fled to Turkey over the past week, many telling of villages burnt and captives beheaded.

"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 at the U.N. General Assembly on Wednesday.

KURDS HALT ISLAMIC STATE ADVANCE

One danger the U.S.-led campaign has in Syria is the lack of strong allies on the ground. Washington remains hostile to the Assad government. It wants other Syrian opponents of Assad to step into the breach as Islamic State is pushed back, but such "moderate opposition" groups have had limited success.

One group that has fought hard against Islamic State on the ground in Syria has been the Kurds, who control an area in the north but complain they have been given no support from the West.

On Thursday, two Kurdish officials said Kurdish forces had pushed back the advance by Islamic State fighters toward the border town of Kobani in overnight clashes. Fighting near the town in recent days had prompted the fastest exodus of refugees of the entire three-year-old Syrian civil war.

Islamic State, which launched a fresh offensive to try to capture Kobani more than a week ago, concentrated its fighters south of the town for a push late on Wednesday, but Kurdish YPG forces repelled them, the Kurdish officials said.

"The YPG responded and pushed them back to about 10 to 15 km (6-9 miles) away," Idris Nassan, deputy minister for foreign affairs in the Kurdish administration in the area, told Reuters by telephone.

Ocalan Iso, a Kurdish defense official, confirmed that YPG forces had stemmed Islamic State's advances south of Kobani, known as Ayn al-Arab in Arabic.

Islamic State fighters also remain to the east and west of the town and fighting continues in the south.

Near Damascus, Assad's Syrian army overran rebels in a town on Thursday, strengthening the Syrian leader's grip on territory around the capital.

Assad's forces, backed by the Lebanese Shi'ite movement Hezbollah, have been gradually extending control over a corridor of territory from Damascus to the Mediterranean coast.

Many Syrian activists and rebels have criticized the United States for focusing on striking Islamic State and other militant groups while doing little to bring down Assad.

Iraq's prime minister told reporters that he conveyed to Syria a message from Washington that U.S. strikes would target Islamic State militants rather than Assad's government.

"What they emphasized is that their aim in Syria is not to destabilize Syria, is not to have a threat of Syrian sovereignty, is not to attack the regime in Syria, but rather to diminish the capabilities of Daesh (and other) terrorist organizations," Abadi said, referring to Islamic State.

FRENCH RESOLVE

The death of French tourist Gourdel, who was beheaded in Algeria 24 hours after an ultimatum was given to France to halt attacks in Iraq, appears to have toughened Paris' resolve.

France said its jets struck four hangars belonging to Islamic State and containing military equipment near the Iraqi city of Fallujah, a stronghold of Islamic State and other Sunni militants just west of Baghdad.

So far, European allies have not joined Washington in strikes in Syria, but French Defense Minister Jean-Yves Le Drian said "the question is on the table."

(Additional reporting by John Irish, Julien Ponthus and Andrew Callus in Paris, Sylvia Westall in Beirut, Nicolas Bertin in Paris; Mark Hosenball, Ian Simpson and Julia Edwards in Washington, and Frank McGurty, Jonathan Allen, Scott Malone and Steve Holland in New York; Writing by Giles Elgood and Peter Graff; Editing by Will Waterman, Peter Cooney and Jonathan Oatis)