(CNN) -- Al Qaeda leader Ayman al-Zawahiri exhorted his fighters to find out who killed his chief representative in war-torn Syria, where militants have had a strong presence and strong rivalries amid the country's civil war.
Two suicide bombers in Aleppo killed Khaled al-Suri in February when they blew themselves up inside a compound used as a headquarters. Al-Zawahiri, in a recorded message posted on a militant website and uploaded on YouTube, deplored the action and called it "sedition."
"All Muslims should not help those who blow up the headquarters of the holy fighters or those who send them car bombs and human bombs, and to stop supporting it in any way," the speaker said.
CNN could not independently confirm that al-Zawahiri made the remarks. But his comments in the past have shown up on the same militant website.
Syria, now in its fourth year of civil warfare, has been the site of what many have called a deadly proxy war between Shiite and Sunni militants.
Iran has backed Bashar al-Assad's government in Syria, and Hezbollah, the Lebanese Shiite group, has fought in support of the government.
Sunni governments, such as Qatar and Saudi Arabia, have supported rebels. Islamic militants, such as al Qaeda-linked groups, have taken up arms in the country.
Rivalry and fighting have emerged between extremist groups in Syria, such al Qaeda-linked Nusra Front and the Islamic State of Iraq and Syria. As a result, al-Zawahiri urged all Muslims and mujahedeen to refrain from infighting.
"Whoever commits such sins should remember that he is fulfilling for the enemies of Islam what they were unable to achieve on their own with all their resources," al-Zawahiri said.
Well over 100,000 people have been killed and millions displaced in the civil war, the United Nations says.