WASHINGTON — A United States citizen working in Syria with a militant group backed by Al Qaeda conducted a suicide bombing there Sunday, in what is believed to be the first time an American has been involved in such an attack, American officials said Wednesday.

The suicide attack first surfaced on Tuesday in Twitter messages from the Nusra Front, an Islamist extremist group in Syria aligned with Al Qaeda in the fight against the government of President Bashar al-Assad of Syria.

American officials, speaking on the condition of anonymity because intelligence matters were involved, declined to identify the American or provide any information about him. NBC News first reported that American government officials had confirmed the bomber was an American.

Syrian activists and jihadist social media sites reported that the American went by the name Abu Huraira al-Amriki and carried out the suicide truck bombing in the northern province of Idlib.