The State Department did not identify the man, said to have been behind a truck bombing on Sunday in the northern province of Idlib, but acknowledged concerns about the flow of foreign fighters into Syria.
"I can confirm that this individual was a US citizen involved in a suicide bombing in Syria. We don't have further information beyond that to share at this time," State Department spokesperson Jen Psaki said.
The man went by the alias Abu Hurayra Al-Amriki, which translates as "The American," she added.
Psaki agreed this is believed to be the first such suicide bombing carried out by an American since 2011, the start of a war in which more than 162,000 people have been killed.
Estimates of the number of foreign fighters who have flooded into Syria in the past three years range from between 9,000 to 11,000, with most believed to have come from neighboring countries.
Psaki could not give precise figures of how many Americans may be among them, although the New York Times said about 100 Americans are believed to have traveled to Syria, mainly to join the rebels fighting to oust President Bashar al-Assad.
"We're engaged in a focused outreach effort with key partner governments regarding our shared concern over the flow of foreign fighters to the Syrian conflict," Psaki said.
Concerns are mounting among Western powers that -- like Afghanistan in the past -- Syria is becoming a training ground for a generation of militants, who will return as battle-hardened veterans ready to carry out attacks both at home and abroad.
"We are closely tracking and closely working with our partners and allies about our concern about the growth of foreign fighters and the growth of extremism," Psaki said, only days after US President Barack Obama unveiled a new $5 billion fund dedicated to the fight against terrorism.
Reports that an American had been involved in the Idlib bombing have been circulating for days. The New York Times, citing law enforcement officials, said he was aged in his 20s, of Middle Eastern descent and from the southern state of Florida.
A Syrian fighter from the Al-Nusra Front told the Times that Abu Hurayra was an Arab-American who spoke only poor Arabic, but was dedicated to the jihadist cause.
"He was a generous, brave, tough man, always on the front lines in battles," the fighter told the US daily.
"When his turn came up" to carry out a suicide bombing, he "was very happy, because he will meet his God after that," the fighter, who called himself Abu Abdulrahman, told the Times via Facebook.
A video released by Al-Nusra supporters, which was first reported on by the private terrorism watchdog SITE Intelligence Group, shows a huge explosion and footage of a young, bearded man cradling a cat and identified as the attacker.
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@Truth: I mentioned them.
Why? These Jihad supporters recruit mentally disturb and vulnerable individual so they can brain wash these poor fellas. Very sad indeed.
Unemployment is already pretty high in the middle east... the last thing Arab men need is for Americans to come and take their jobs...
You can bet money that they will investigate this guys history and associates in an attempt to determine how he was radicalized. Whatever mosque/Imam's he used are going to come under scrutiny.
@Ajeet
One cannot strip citizenship and deport a deceased.
Strip them of citizenship and deport them
I can only feel S a d with a capital S. Poor midguided pet.