WASHINGTON — U.S. intelligence and law-enforcement agencies have identified nearly a dozen Americans who have traveled to Syria to fight for the militant group Islamic State.

As the group has seized large expanses of territory in recent months, it has drawn more foreign men to Syria. As a result of the increasing numbers of men, Islamic State also is recruiting foreign women as jihadi wives.

Islamic State has become more attractive to would-be militants because, unlike al-Qaida, it has seized territory that it rules by strict Islamic law. “ISIS is able to hold itself up as the true jihad,” a senior U.S. official said, using the abbreviation for another name for Islamic State. “They’re saying: ‘Look at what we are doing, what we’re accomplishing. … We’re not just talking about it. We’re doing it.’”

Islamic State’s attraction to some is based on its reputation for brutality. On Thursday, that reputation grew worse when it was revealed that it had waterboarded four hostages early in their captivity, including U.S. journalist James Foley, who was beheaded this month.

Overall, U.S. intelligence officials said the number of Americans who had joined rebel groups in Syria — not just Islamic State — had nearly doubled since January. The officials believe that more than 100 Americans have fought alongside groups there since the civil war began three years ago.

U.S. officials said they had been able to identify Americans fighting for Islamic State based on intelligence gathered from travel records, relatives, intercepted electronic communications, social-media postings and surveillance of Americans overseas who had expressed interest in going to Syria.

Far more Europeans have joined the fight against President Bashar Assad, more than 1,000, according to many estimates. The British government has identified about 500 citizens who have gone to Syria, according to a senior British official. About half have returned to Britain, and a few have died on the battlefield, the official said.

Senior U.S. officials acknowledge that as the conflict in Syria and Iraq drags on, it is becoming more difficult to track Americans who have traveled there. In the latest example of how difficult, the FBI on Thursday was attempting to verify reports that two more Americans had been killed fighting for Islamic State in Syria.

Since the beginning of the Syrian conflict in 2011, at least four Americans have died fighting for rebel groups, including Douglas McCain, 33, a Minnesota man who was fighting for Islamic State when he was killed last weekend by a rival U.S.-backed group.

Another challenge intelligence and law-enforcement authorities say they face is that unlike previous conflicts, the Americans who have traveled to Syria to fight have little in common. The conflict has attracted men and women, including some who were raised as Muslims and others who converted from Christianity, and they have come from different parts of the United States.

The territorial gains by Islamic State and its attempt to govern towns and cities in eastern Syria and western Iraq have forced it to recruit foreigners, not just for the battlefield.

In Minneapolis, Abdirizak Bihi, director of the Somali Education and Social Advocacy Center, said young Somali women were being recruited by violent Islamist groups to support Syrian militants. Bihi said that despite efforts to combat the recruiting, multiple Somali families in the city have “lost their girls to Syria.”