The confirmation came after an American citizen named Nicole Mansfield of Flint was killed last week in the company of Syrian terrorist in Idlib Province.
Former US soldier Eric Harroun also was arrested on his return to the United States in March and charged with conspiring to use a rocket-propelled grenade in Syria.
Investigators said he acknowledged fighting with terrorist groups, including the al-Qaeda-linked Nusra Front.
US officials and experts say there is information on other Americans who have gone to Syria to fight.
Foreign militants from other countries are also flocking to join the terrorists in Syria.
At most, said one expert who monitors US militant activities and websites, up to 20 Americans may have gone to Syria. Other experts put the number at half that many.
A US security official said there was no formal US government tally of how many Americans had gone to Syria to fight the Syrian government but that the best current estimate was “a handful.”
According to a new study about foreign fighters reported killed in the Syrian conflict, the largest contingents of foreigners participating in the conflict come from nearby countries riven by conflict as a result of the so-called “Arab Spring.”
The study, a collaboration between experts at the Washington Institute for Near East Policy and Flashpoint Global Partners who monitor extremist Internet sites, analyzes the national origins of 280 foreign fighters reported to have died fighting with rebels in Syria between July 2012 and May this year.
The study found that among the 280 dead fighters whose cases were examined, the largest single contingent - 60 - came from Libya, and the second-largest group - 47 - came from Tunisia, countries where authoritarian governments were toppled by popular uprisings during the "Arab Spring."
The third largest group of nationals among the dead foreign fighters in Syria consisted of 44 Saudis, followed by 32 Jordanians, 27 Egyptians, 20 Lebanese, seven Russians, five Kuwaitis, five Chechens, and three Iraqis.
The study said the death toll also included single foreign fighters from countries including Denmark, France, Uzbekistan, Ireland, Morocco, Algeria, Kosovo, Turkey, Bulgaria, Britain and the United States, as well as three from Dagestan, three from the United Arab Emirates and two Australians.
The study authors also stated that the “lion's share” of foreign fighters who died in Syria did so while fighting with the terrorists Nusra Front, known in Arabic as Jabhat al-Nusra.
Security officials in the United States and Europe say recent official reporting on the current involvement of foreign fighters in Syria indicates that the death toll underestimates the involvement in the conflict of British citizens.
European security officials have estimated that between 70 and 100 Britons are currently in Syria, most of them fighting with the Nusra Front or other terrorist groups. European and US officials have also said there has been a recent surge in fighters going to Syria from Chechnya and Dagestan in Russia.
US and European officials said they saw few signs that well-organized networks recruited foreign fighters for Syria and arranged for their travel and contacts inside Syria. Instead, said the officials, most foreigners seeking to join anti-government forces traveled on their own to Turkey or Lebanon and then made their own way through loosely guarded border posts.