BAGHDAD — 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.

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 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.

“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 IS 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 President 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 IS 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 IS militants swept through a swath of Iraq in June, proclaimed a “caliphate” ruling over all Muslims, slaughtered prisoners and ordered Shi’a and non-Muslims to convert or die.

A third night of air raids by the United States and Arab allies targeted IS-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 the group’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.

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 IS 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.

IS, 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 the extremists’ advances south of Kobani, known as Ayn al-Arab in Arabic.

IS militants also remain to the east and west of the town and fighting continues in the south.