An image from Jihadist media outlet Welayat Raqa on July 25, 2014, allegedly shows members of the Islamic State militant group raising their index finger as they loot ammunition from a Syrian army base. (Via AFP/Getty Images)

The radical Islamist State terrorist group has pushed to establish cells outside Iraq and Syria, including in Europe, even as it seeks to solidify its gains in the Middle East, U.S. intelligence officials said Thursday.

The group’s efforts to position operatives in Western countries is considered evidence of the al-Qaeda offshoot’s determination to mount terrorist plots against the United States and its allies.

“We have seen an expansion of its external terrorism ambitions” that parallel its aggressive moves in the Middle East, a senior U.S. intelligence official said at a briefing for reporters on the threat posed by the Islamic State.

The official said the organization has attracted thousands of foreign fighters, including Western passport holders who now rank among its forces in Iraq. Some of its recruits from Europe are leaving with orders to go home and start cells, the official said.

He and others participating in the briefing spoke on the condition of anonymity to describe U.S. intelligence assessments of the Islamic State, a group that was established as an al-Qaeda affiliate in Iraq but this year declared itself a rival of that terrorist network.

The officials would not provide details on the locations or composition of the Islamic State’s cells in Europe and stopped short of saying that the group has advanced beyond its stated intentions to attack the United States and to execute specific plots.

Still, the officials’ characterization of the group represents the clearest indication to date that U.S. counterterrorism analysts consider it a direct and growing threat to the country.

The account comes as the Obama administration is seeking to contain the Islamic State’s advances in Iraq with a limited campaign of airstrikes that are designed to protect refugees and U.S. personnel in the region but not dismantle the terrorist group.

Obama said Thursday that the air campaign will continue even after ending an Islamic State siege on a mountaintop where thousands of members of the minority Yazidi sect were stranded, but he made no mention of the potential danger the Islamic State poses to the United States and its allies.

U.S. intelligence officials said the group has grown rapidly in numbers and strength since taking control of the northern Iraqi city of Mosul in June and noted that it is in position to bolster its substantial cash holdings with sales of oil.

The officials said that U.S. intelligence analysts are revising estimates of the group’s size and that it has grown substantially beyond the 10,000 or so fighters it had just several months ago.

“What they have . . . is momentum,” one intelligence official said.

The group is not believed to have the technical expertise or experience for elaborate plots or sophisticated explosives, the officials said, contrasting it with the al-Qaeda affiliate in Yemen, where a skilled bombmaker, Ibrahim al-Asiri, has been instrumental in a series of near-miss attacks involving hidden devices aboard airplanes.

But the Islamic State is better funded, highly organized and has access to a steady stream of recruits from countries including Britain and France. The officials cited as an example of the group’s reach the arrest of an Islamic State-linked figure this year after an attack on a Jewish museum in Belgium.

The group’s rapid expansion makes it vulnerable to overreach, officials said, and its leader’s decision to declare himself head of a reinstated Islamic caliphate has both boosted its reputation with some Islamists and prompted others to condemn its arrogance.

The Islamic State’s declaration “has sparked serious debates within the extremist cleric community,” a U.S. intelligence official said. Turki al-Binali, an ultra-radical cleric in Bahrain, has declared his support to the Islamic State. Others, including Abu Muhammad al-Maqdisi, an influential Jordanian cleric, have opposed the group. The latter was a mentor to Abu Musab al-Zarqawi, a Jordanian fighter who established al-Qaeda’s presence in Iraq after the U.S. invasion a decade ago and was killed in a 2006 strike.