Syrian state-run TV reported Friday that the leader of a powerful al-Qaida-linked rebel group has been killed — a claim that if confirmed would be a huge blow to fighters trying to topple President Bashar Assad. At least one rebel commander denied the report.

Abu Mohammad al-Golani heads Jabhat al-Nusra, also known as Nusra Front, which has emerged as one of the most effective among the rebel groups fighting Assad.

The Britain-based Syrian Observatory for Human Rights, which closely monitors the fighting in Syria, said senior Nusra Front leaders contacted by activists in Latakia and the eastern Deir el-Zour province denied that al-Golani had been killed.

Other Nusra Front sources said they could not confirm or deny the report “because contact with al-Golani was cut,” the Observatory said in a statement. A rebel commander in a Damascus suburb said he believed al-Golani was “alive and well” based on his contacts with other fighters, including those from Nusra Front. He declined to elaborate or be identified for security concerns.

The report comes as the fragmented rebels have suffered significant losses on the battlefield.

Syrian troops killed at least 40 opposition fighters, including Nusra Front members, earlier Friday in an ambush near Damascus, the government said.

Assad’s forces backed by Lebanese Hezbollah gunmen also seized control of a rebel ammunition supply route on a highway linking the capital to its eastern suburbs — part of a blistering government offensive to bolster its position amid an international push for peace talks.

State TV said al-Golani was killed in the coastal province of Latakia. It did not say when or give other details. News of his death was not mentioned in the main headlines of the TV’s late night news bulletin.

Al-Golani, who fought previously in Iraq, is a shadowy figure who is believed to have spent time recently in rebellious suburbs south of Damascus. Rebels have also gained footholds in the mountainous regions of Latakia, which is largely loyal to Assad, and he may have gone there to direct the fighting.

The Nusra Front is on a U.S. State Department list of terrorist organizations. The group has claimed responsibility for numerous suicide bombings against government targets.

Al-Golani gained prominence in April when he rejected an attempted takeover of the group by Abu Bakr al-Baghdadi, the head of the Islamic State of Iraq, revealing a growing rift within al-Qaida’s global network.

Al-Golani at the time distanced himself from claims that the two groups had merged into a group called the Islamic State of Iraq and the Levant (ISIL). Instead, he pledged allegiance to al-Qaida leader Ayman al-Zawahri.

He said that al-Baghdadi’s announcement of the merger was premature and that his group will continue to use Jabhat al-Nusra as its name.

The group is more popular in Syria than the ISIL, which is largely made up of foreign fighters and has been criticized for its brutality and for trying to impose a strict version of Islamic law in areas under its control.

Al-Golani’s death could strengthen ISIL at a time of growing infighting between al-Qaida extremists and more moderate rebels from the Free Syrian Army.

(story can end here)

Assad’s forces have been gaining ground in rebel-held areas around the capital, the seat of his power, and have made progress against outgunned and fragmented fighters in several areas.

On Friday, the state-run news agency SANA said 40 rebels died in the ambush near Otaiba, adding that soldiers seized a large arms cache, including anti-tank rockets.

The area is part of a region known as Eastern Ghouta, which was the scene of a chemical weapons attack in August believed to have killed hundreds.

The state-run Al-Ikhbariya television station broadcast footage showing more than a dozen bodies near the largely dried-out Otaiba lake, some wearing flak jackets strapped with ammunition. Automatic rifles and hand grenades lay nearby.

An unidentified Syrian army officer in the area told Al-Ikhbariya that foreign fighters were among the dead and that the ambush followed an intelligence tip.

The Observatory said at least 24 fighters, some of them foreign, were killed in the ambush, but it gave no further details and the differing death tolls could not be immediately reconciled.

The offensive coincided with an international push for a peace conference to be held in Geneva. Both sides want to bolster their position on the ground ahead of the talks, expected next month. No final date has been set, however, and it is unclear whether the sides will reach an agreement on the agenda.