They told a court that Eric Harroun, 30, could face execution if his actions are found to have caused a death.
During interviews with the FBI, Mr Harroun allegedly said he had shot 10 people in Syria, but was not sure if he had killed anyone.
He was denied bail at Monday's hearing in Alexandria, Virginia.
Mr Harroun, who has been charged with using a weapon outside the US, allegedly fought in Syria with the al-Nusra Front.
The group wanted Mr Harroun, who served in the army from 2000-03, to act as their spokesman, but he declined to accept the role, prosecutors added.
Source: Quilliam Foundation
New insight into Nusra Front
Profile: Syria's al-Nusra Front
Defence lawyers said the al-Nusra Front shares the US government's opinion that Syrian leader Bashar al-Assad must go.
"It is extremely unusual for the US to charge a person who is fighting in a manner that is aligned with US interests," said public defender Geremy Kamens.
According to a criminal complaint, Mr Harroun is accused of crossing into Syria in January 2013 and fighting alongside members of Jabhat al-Nusra (the Nusra Front) against Mr Assad's forces.
He allegedly fought as part of a group using rocket-propelled grenades, but is not charged with providing material support to a terror group.
Mr Harroun was medically discharged from the Army in 2003 after a car accident and lived in Phoenix, Arizona.
He never served overseas, according to an Army spokesman.
In March, the FBI spoke to the former soldier in three voluntary interviews at the US consulate in Istanbul.
Officials say he told them he wanted to fight with the Free Syrian Army against the Assad regime and allegedly confirmed he had been fighting with al-Nusra.
A former US soldier who allegedly fought Syrian government forces with a group linked to al-Qaeda could face the death penalty, prosecutors have said.