Kashmiri demonstrators hold up Palestinian flags and a flag of the Islamic State during a demonstration against Israeli military operations in Gaza, in downtown Srinagar on July 18, 2014. (Tauseef Mustafa/AFP/Getty Images)

U.S. spy agencies have begun to see groups of fighters abandoning al-Qaeda affiliates in Yemen and Africa to join the rival Islamist organization that has seized territory in Iraq and Syria and been targeted in American airstrikes, U.S. officials said.

The movements are seen by U.S. ­counterterrorism analysts as a worrisome indication of the expanding appeal of a group known as the Islamic State that has overwhelmed military forces in the region and may now see itself in direct conflict with the United States.

“Small groups from a number of al-Qaeda affiliates have defected to ISIS,” as the group is also known, said a U.S. official with access to classified intelligence assessments. “And this problem will probably become more acute as ISIS continues to rack up victories.”

The influx has strengthened an organization already regarded as a menacing force in the Middle East, one that has toppled a series of Iraqi cities by launching assaults so quickly and in so many directions that security forces caught in the group’s path have so far been unable to respond with anything but retreat.

U.S. officials attribute the Islamic State’s rapid emergence to factors both psychological and tactical. Its core group of fighters honed their skills against the armies of Syrian President Bashar al-Assad and the United States when it occupied Iraq. The group has used raids and ransoms to stockpile weapons and cash. And its merciless reputation triggered rampant defections among Sunni members of Iraq’s security forces already disenchanted with the Shiite-led government in Baghdad.

1 of 42 Full Screen Autoplay Close Aug. 10, 2014 Aug. 9, 2014 Saturday Aug. 7, 2014 Thursday Aug. 5, 2014 Tuesday Aug. 4, 2014 Monday Aug. 3, 2014 Sunday Skip Ad × U.S. airdrops aid to Iraqis fleeing Islamic State forces View Photos Iraqi civilians fled as panic set in with the retreat of Kurdish forces from Islamic State militants. Meanwhile, President Obama approved humanitarian airdrops to besieged Iraqis escaping the Sunni extremists. Caption After being stranded on the parched Mount Sinjar for almost a week and then walking 12 miles to seek refuge in Syria, some of the persecuted civilians were able to safely return to the Kurdish region of Iraq with the help of Syrian Kurdish forces. Aug. 12, 2014 Displaced Iraqis from the Yazidi community gather for food at the Nowruz camp in Derike, Syria. Khalid Mohammed/AP Buy Photo Wait 1 second to continue.

Even before its assault on Kurdish territories in northern Iraq this month, analysts said the Islamic State had shown an almost impulsive character in its pursuit of territory and recruits, with little patience for the elaborate and often time-consuming terror plots favored by al-Qaeda.

Counterterrorism analysts at the CIA and other agencies have so far seen no indication that an entire al-Qaeda node or any of its senior leaders are prepared to switch sides. But officials said they have begun watching for signs of such a development.

The launching of U.S. airstrikes has raised new questions, including whether the bombings will hurt the Islamic State’s ability to draw recruits or elevate its status among jihadists. “Does that increase the spigot or close it?” said a senior U.S. counterterrorism official, who, like others, spoke on the condition of anonymity and noted that U.S. military operations in Afghanistan and elsewhere have crippled al-Qaeda but also served as rallying cries against the United States.

Longer-term, U.S. officials expressed concern that the Islamic State, which so far has been focused predominantly on its goal of reestablishing an Islamic caliphate, may now place greater emphasis on carrying out attacks against the United States and its allies.

President Obama was careful to depict the strikes as part of a humanitarian mission to protect endangered Iraqis, including members of a Christian sect, encircled with scant supplies on a northern Iraq mountaintop. Obama also referred to the presence of U.S. personnel in the region and stopped short of authorizing a broader assault against the Islamic State.

Still, the strikes triggered widespread calls for retaliation among militant groups online. A prominent figure on a well-known jihadist forum, Shumukh al-Islam, wrote Friday that the airstrikes should prompt fighters to unite against the United States.

“The mujahideen must strike and seek to execute proactive operations in their own home, America, to discipline America and its criminal soldiers,” the jihadist, Abu al-Ayna al-Khorasani, wrote, according to the SITE Intelligence Group, which monitors militant postings.

U.S. officials said the defections to the Islamic State have come primarily from al-Qaeda in the Arabian Peninsula, the Yemen-based group that has launched several bombing plots targeting the United States, and al-Qaeda in the Islamic Maghreb, which had seized territory in northern Mali before facing strikes carried out by France last year.

“It’s not to the point where it’s causing splintering within the affiliates,” said the senior U.S. counterterrorism official. But the defections have accelerated in recent months, officials said, and also involve fighters from groups in Libya and elsewhere that are not formally part of al-Qaeda.

U.S. officials estimate that the Islamic State has as many as 10,000 fighters, including 3,000 to 5,000 from countries beyond its base in Iraq and Syria. Its ranks have swelled with the emergence of the civil war in Syria — a country relatively easy to reach from both in the Middle East and Europe — as a larger magnet for jihadis than Afghanistan or Iraq were. The group has also attracted critical support from disenfranchised Sunni residents in Mosul and other Iraqi cities, civilians who have lost patience with the government of Iraqi President Nouri al-Maliki but may not embrace the hard-line agenda of the Islamic State.

The group has not been linked to any known plot against the United States, but Director of National Intelligence James R. Clapper Jr. testified in January that the group “does have aspirations for attacks on the homeland.”

U.S. officials have said about 100 Americans have either traveled to Syria or tried to. Among them was a former Florida resident, Moner Mohammad Abusalha, who returned undetected to the United States for several months this year before departing again for Syria and detonating a suicide bomb. Abusalha was not tied to the Islamic State, but officials believe that as many as a dozen Americans have linked up with the group.

The Islamic State traces its origin to al-Qaeda in Iraq but broke from the terrorist network this year after being criticized for its tactics — including the slaughter of civilians — and refusing instructions to cede the fight in Syria to a separate al-Qaeda ally known as al-Nusra.

Since then, the Islamic State has amassed arms, cash, fighters and territory at a breathtaking rate. In July, the group’s leader, Abu Bakr al-Baghdadi, took the pulpit at the largest mosque in Mosul, declaring himself the “caliph” of the Muslim world and urging followers to flock to his organization.

In doing so, Baghdadi fulfilled an ambition articulated by his predecessor, Abu Musab al-Zarqawi, who was killed by U.S. forces in Iraq in 2006. It also marked a significant departure from the al-Qaeda playbook.

Al-Qaeda’s commander in Yemen, Nasir al Wuhayshi, has written letters to subordinates cautioning against prematurely declaring Islamic rule even in small villages — in part out of fear that failing to hold territory or enforce Islamic law would lead the group to lose face with the local population.

Baghdadi’s lack of restraint appears to have expanded his appeal, according to U.S. officials who said the group’s expanding territory, aggressive reputation and roster of experienced fighters account for its momentum.

“They are demonstrating just how advantageous it is to a ­terrorist-insurgent group to be fighting in the field for years and years as they have been in Iraq and Syria,” said Daniel Benjamin, a professor at Dartmouth University who previously served as the top counterterrorism official at the State Department.

“Their skill at maneuver is really kind of extraordinary compared to groups you would compare them to,” including al-Qaeda’s affiliates in Yemen and Mali, Benjamin said. “They are not constrained by that fear of failure other al-Qaeda groups have shown,” he added, or the group’s tendency to “spend years preparing single attacks.”

Julie Tate contributed to this report.