Eric Harroun, a 30-year-old army veteran from Phoenix, Arizona, is alleged to have joined a squad of guerrilla fighters from the Al-Nusra Front, training to use rocket-propelled grenades against Syrian government forces.
Although Washington also opposes Bashar al-Assad's regime, US authorities regard Al-Nusra as a front for al-Qaeda's offshoot in Iraq, an Islamist group which it designates as an international terrorist organisation.
According to a statement from the Justice Department, Harroun has been charged with "conspiring to use a destructive device outside of the United States", an offence that carries a maximum penalty of life in prison.
News that makes sense
Your trusted source for staying up-to-date with the world around you. Get free daily news updates and analysis, straight to your inbox.
The statement did not say when, where or how Harroun had been arrested, nor how he was brought to Thursday's hearing in a federal court in Alexandria, Virginia, to hear the charges laid against him.
Between 2000 and 2003, Harroun served with the US Army.
In January of this year, he crossed into Syria and joined a group of Al-Nusra rebels, prosecutors allege. He also posted online videos of himself training with a rocket-propelled grenade launcher or RPG.
"Harroun allegedly participated in attacks led by the Al-Nusra Front and was part of an RPG team, for which he carried anti-personnel and anti-armour rockets," the statement said.
The case is being investigated by the Federal Bureau of Investigation.

