New York Mayor Bill de Blasio (centre), with FBI assistant director-in-charge George Venizelos (left) and New York City Police Commissioner William Bratton, addresses a news conference at the Union Square subway station in New York on Thursday.  Bratton said that the department boosted police presence on subways and city streets after the Iraqi prime minister’s warning of a potential threat to transit systems from Islamist militants.

AFP/Washington

US authorities believe the Islamic State jihadists will try to stage an attack in America in reprisal for US-led air strikes in Syria, the head of the FBI said.

"There's no doubt," director James Comey told reporters on Thursday. "They want to do things in retaliation for America's actions or its allies."

He added: "I'm sure they'll desire to find a way to strike here."

The US and Arab allies began bombing Islamic State targets on Tuesday, and overnight Wednesday began hitting oil installations controlled by the group in a bid to cut off its money supply.

Since August 8 the United States has conducted around 200 air strikes against the group in Iraq.

"The logic is that if they aspire to be the leader of the global jihad you don't get there without striking America," he said.

"I have absolutely no doubt that if they have the capability, which I don't believe they have, to conduct some sophisticated simultaneous attack, they would," Comey told journalists.

He gave no details on what kind of attack the Americans fear, but said the goal would be to "find any way to kill or brutalise innocent Americans."

Comey also said the US has identified the masked Islamic State militant who murdered two kidnapped American journalists in separate videotaped beheadings.

"We believe we have identified the executioner," Comey told reporters at a briefing in Washington. "I won't tell you who it is."

In separate videos released by the extremists, a man with a black mask and speaking with a British accent says the killings are in reprisal for air strikes against the group in Iraq.

The two American journalists wear jump suits of the same bright orange worn by inmates at the US prison in Guantanamo.

The group has also claimed responsibility for the later killing of British air worker David Haines.

Comey also said he was concerned about people being recruited in the US to go and fight as jihadists for the Islamic State group in Syria or Iraq.

He said about a dozen Americans are now fighting with terror organisations.

"I'm concerned that ISIL will motivate and radicalise people in the US," he said, using another acronym for the Islamic State group.

Asked about a strike this week against the Khorasan Group, which is close to Al-Qaeda, Comey said he was "operating under the assumption that they're still intact."

The US has not been able to confirm how badly the group was hit or if its leader was killed in the attack this week.

US officials said the group had been plotting an attack in the US.

"That group was at the top of my list of the things that I worry about," Comey said.

"Because it's a collection of very, very, very bad and experienced terrorists ... into which we don't have complete visibility."

"They were working toward an attack. Hard to say whether it is tomorrow, three weeks from now or three months from now," he said

"But I have to work on the assumption that it is tomorrow."