Al-Qaeda's Syrian branch was left reeling after its military chief was killed in an apparent army air strike.
The death of Abu Humam al-Shami on Friday, who trained in Afghanistan alongside the September 11 plotters, has left doubts over the future of the most powerful group opposing both President Bashar al-Assad and Islamic State.
The Nusra Front controls wide parts of northwestern Syria, and became one of the most powerful anti-Assad forces after it split from Islamic State following the group's increased radicalisation.
"Everyone is still trying to figure out what happened yesterday. Two of the leaders were killed in coalition air strike around a week ago," said the commander of a rival Islamist brigade in northern Syria.
"Abu Humam's killing is very significant ... But this group has built itself in a way that when one of its leaders is killed it won't leave a gap."
The Syrian military said it had carried out Thursday's attack, which also killed a number of other Nusra leaders. A Syrian military source said the headquarters had been struck from the air.
Jihadist sources had initially said Thursday's blast was the result of an air strike by a US-led coalition that has been bombing Islamic State in Syria.
However, the coalition denied mounting any strikes in the province in the preceding 24 hours. While the coalition has focused on Islamic State, the United States has occasionally also targeted Nusra Front figures.
Rami Abdulrahman, who runs the Syrian Observatory for Human Rights which tracks the war in Syria using sources on the ground, said Abu Humam was more important than the Nusra Front's overall leader, Abu Mohamad al-Golani.
"It will take time before the impact of his death becomes clear", Abdulrahman added.
It is currently a major force in fighting against government forces and allied militia around the northern city of Aleppo, where its fighters took part in a major attack on a state security building this week.
It is also fighting in southern Syria, where the army and allied militia have launched a major offensive, and its fighters have frequently clashed with Islamic State in a number of locations.
The biography released by the Nusra Front says Abu Humam served as a military trainer for al Qaeda in Afghanistan at one of its main training camps in Kandahar. Later, in Iraq, he trained leaders for Abu Musab al-Zarqawi, the founder of the al Qaeda wing that morphed into Islamic State.