A senior military commander and founding member of the Syrian rebel group Jabhat Fatah al-Sham, formerly known as the Nusra Front, has been killed by a US air strike that hit a meeting of the group's leaders, rebels said.

Abu Hajer al-Homsi, whose real name was Abu Omar Saraqeb, was killed in a raid on a rural part of Aleppo province on Thursday, which was most likely carried out by a US fighter jet, a rebel source told the Reuters news agency.

Another source told Reuters that the rebels were at a hideout in the village of Kafr Naha when the strike hit them.

The AFP news agency also reported Saraqeb's death, citing rebels.

Few other details emerged, but a photo of another top leader known as Abu Muslim al-Shami was circulated on social media, showing him alive to refute reports he had also been killed.

The leader of the group, Abu Mohammad al-Jolani, was not believed to be at the meeting.

Since a US-led coalition began launching raids on groups in Syria and Iraq in September 2014, bombing raids have frequently targeted Nusra Front figures in Syria, also resulting in the deaths of scores of civilians. But Thursday's attack marked the first time a key figure had been targeted since the group changed its name.

The Nusra Front announced in July that it was ending a relationship with al-Qaeda and changing its name to Jabhat Fatah al-Sham in an attempt to appeal to those Syrians who had misgivings about its links with al-Qaeda, and the presence of foreign fighters in its ranks.

The move was dismissed by Washington, which said it would continue to consider the group "terrorists".

READ MORE: The ramifications of the Nusra's split from al-Qaeda

Battle for Aleppo

US Secretary of State John Kerry and Russian Foreign Minister Sergei Lavrov have been trying to reach a deal on deeper cooperation on Syria, particularly in their raids on the Islamic State of Iraq and the Levant (ISIL, also known as ISIS).

The two were expected to meet in the Swiss capital Geneva on Friday for face-to-face talks.

The talks "will focus on reducing violence, expanding humanitarian assistance for the Syrian people, and moving toward a political solution needed to end the civil war," US State Department spokesman John Kirby said in a statement.

The meeting comes as government forces make major gains on the outskirts of the divided city of Aleppo.

Taking Aleppo would be the biggest victory for Syrian President Bashar al-Assad in five years of fighting, and would demonstrate a dramatic shift of fortunes in his favour since Russia joined the war on his side last year.

The Syrian conflict began as a mostly unarmed uprising against Assad in March 2011, but quickly escalated into a full-blown civil war.

More than 280,000 Syrians have been killed during the war, 4.8 million have fled the country, and 6.6 million have become internally displaced by the violence, according to the UN.

The UN special envoy to the country, Staffan de Mistura, has estimated that more than four million people may have been killed, but that is not an official figure.