The US-backed Syrian Democratic Forces (SDF) captured two Americans in Syria who are suspected of being ISIS fighters, the militia said in press release Sunday.
The militia identified the men as Warren Christopher Clark (Abu Mohammad al-Ameriki) and Zaid Abed al-Hamid (Abu Zaid al-Ameriki).
The SDF said Clark is originally from Houston, but it gave no specific location in the United States for the other man's origin.
The Americans were captured in a group that also contained fighters from Ireland and Pakistan, SDF said.
A Pentagon spokesman said the incident is under investigation.
"We are aware of open source reports of reportedly American citizens currently in custody who were believed to be fighting for ISIS. However, we are unable to confirm this information at this time," Commander Sean Robertson said.
The White House announced last month that ISIS had been defeated in Syria and the United States would withdraw 2,000 troops from the war-stricken nation.
In October, the US military released another American citizen suspected of being a member of ISIS who was captured in Syria.
The dual US-Saudi national was held by the US military in Iraq without being tried since September 2017. His fate was the subject of a months-long legal battle between the US government and the American Civil Liberties Union.
When he was released, CNN reported that SDF has more than 700 foreign terrorist fighters in custody from some 40 countries.
US officials have encouraged countries to repatriate their citizens in detention in Syria in order to ease the burden on the SDF's detention facilities. But many countries are reluctant to do so because of the difficulty of prosecuting suspected ISIS members based on evidence collected on the battlefield.