ISTANBUL — Nearly a decade ago, Ayman al-Zawahiri — the man who would go on to become the head of Al Qaeda — wrote a letter to his deputy in Iraq, scolding him for beheading hostages and posting videos of their execution online. He explained that although he was in favor of killing the enemy and agreed with the principle of sowing terror, the scenes of slaughter risked turning public opinion against their organization.

His advice was to be more discreet: “Kill the captives by bullet.”

The letter — written in 2005 and recovered by American forces in Iraq — was addressed to Abu Musab al-Zarqawi, the founder of the group that would become the Islamic State, which split off from the Qaeda network earlier this year.

On Monday, Al Qaeda came out publicly against the practice of beheading in a strongly worded interview with one of its field commanders, making clear that the organization founded by Osama bin Laden was more pragmatic and as a result less extreme than its jihadist rival in Syria — which has turned the act of decapitation into a signature of its brutality.

In a 43-minute video, Nasr bin Ali al-Ansi, a military strategist and official of Al Qaeda’s branch in Yemen, is asked whether he condones recent beheadings. He says that although some Qaeda members may have carried out such acts, the organization does not sanction the practice.