A military official says the woman and her son were detained about 10 days ago using fake identification cards.
Both officials refused to give any details about the woman who is believed to be one of the wives of Abu Bakr al-Baghdadi, the group's reclusive leader.
The officials spoke on condition of anonymity in line with regulations.
The military official said today that the woman is a Syrian citizen.
The Lebanese daily As-Safir was the first to break the news, saying they were detained near a border crossing point with Syria. It added that the arrest was in "coordination with foreign intelligence agencies.