Speaking to reporters on Thursday, FBI chief James Comey announced a significant step in the hunt for the "Islamic State" militant responsible for the deaths of American journalists James Foley and Steven Sotloff.

"We believe we have identified the executioner," Comey said.

The masked jihadist (pictured), who appears in both execution videos, had caught the attention of the West. The fact that he spoke with a British accent drew attention to the previously underestimated role of foreign fighters within the extremist group, which now controls larges swathes of land in Iraq and Syria.

However, the FBI chief declined to comment further on the case or indicating the man's nationality.

"I won't tell you who it is," he said.

The Islamist extremist group began the barbaric executions of citizens in August as revenge against countries who are contributing to the US-led air strikes in Iraq and Syria. Foley and Sotloff were their first two victims. They were executed within several weeks of each other. They also beheaded a British aid worker, David Haines, in the weeks that followed.

Video militant from North America?

Comey also commented on new evidence of North American involvement in the jihadist group based on a video released recently.

"There's no doubt that there's someone speaking with a North American-accented English on that video, so that's a big focus of ours right now," Comey said.

Leaders from across the globe have grown alarmed at the swift rise of the jihadist group and particularly of its effective recruitment of men from countries where the Muslim populations form a small minority. These include several major western European countries, the United States, Canada and Australia.

France was its most recent target. Earlier this week, an Algerian group known as Jund al-Khilafah, which is an IS supporter, kidnapped a French citizen while he was hiking. It released the video of his subsequent execution on Wednesday.

Subway plot unconfirmed

Meanwhile on Thursday, Iraq's prime minister, Haider al-Abadi, told reporters on the sidelines at the United Nations that he had evidence of a terrorist plot against the US and France. Iraqi intelligence had received the information from militants they had captured, he said, adding that foreign fighters from France and the US were co-conspirators.

The terrorist plot purportedly targets the subway system in New York City and Paris.

However, government officials from both the United States and France have said they had no information that could confirm the Iraqi prime minister's claim.

kms/jr (AP, AFP, Reuters)