At least three workers trying to repair a hospital in rebel-held eastern Aleppo were killed on Monday when it was hit by a third airstrike inside a week, a local medical official said.

The airstrike killed another four people outside the M10 underground hospital in the besieged enclave, Dr Abu al-Izz of the Syrian American Medical Society, which runs the hospital, said.

The repeated attacks on hospitals in rebel-held Aleppo, apparently by the Syrian and Russian air forces, have drawn condemnation from the UN and western countries and have led the US to threaten an end to its engagement with Russia over the conflict in Syria.

Al-Izz and Ibrahim al-Hajj of the White Helmets rescue organization in eastern Aleppo said the hospital had been hit by a "bunker-buster" bomb - designed to demolish below-ground facilities - which, they said, are dropped by Russian planes.

Russia intervened in Syria a year ago to prop up President Bashar al-Assad's crumbling army.

Its air campaign has allowed government forces to reclaim some territory from both rebels and the Islamic State extremist group, most notably imposing a siege on eastern Aleppo where some 250,000 to 300,000 civilians are now thought to be trapped.

Meanwhile, a key Syrian rebel group with close links to al-Qaeda said one of its senior leaders had been killed in an air strike by a US-led coalition.

The Fatah al-Sham Front, which until July was known as al-Nusra Front and functioned openly as the Syrian branch of al-Qaeda, described Ahmed Salameh, also known as Abu Faraj al-Masri, as a member of its "consultative council."

Salameh was one of two officials appearing alongside Fatah al-Sham's leader Abu Mohammed al-Jaulani in a July video statement that announced the group's renaming and its apparent break from al-Qaeda, experts on the organization said at the time.

Pentagon spokesman Jeff Davis said that US forces were assessing the results of an airstrike that targeted a senior member of "core al-Qaeda" in Syria.

The US has repeatedly targeted al-Nusra leaders as part of its wider air war in Syria, which is mainly focused on the rival jihadist organization that calls itself Islamic State.

It has typically described its strikes as targeting what it calls the Khorasan Group, which it says is composed of senior al-Qaeda leaders who moved to Syria during the conflict.

The strikes on al-Nusra have been unpopular with the Syrian opposition, much of which sees the jihadists as key military allies on the ground regardless of ideological differences.

The jihadist organization's disciplined fighters and willingness to break enemy defences with suicide attacks have led to it playing a key role in major opposition offensives such as an assault in August that allowed rebels to break the siege of eastern Aleppo for several weeks.

Meanwhile, at least 21 Turkish-backed Syrian opposition rebels were killed while combing an area previously held by Islamic State north-east of Aleppo, a monitoring group said.

"While rebels were combing the Turkman Barih village on Sunday, landmines planted by the jihadist movement exploded, also wounding 29 other rebel fighters," the Britain-based Syrian Observatory for Human Rights said.    

However, local Turkish media quoted an army statement as saying "clashes in northern Syria have killed 15 Ankara-backed Free Syrian Army (FSA) fighters in the last 24 hours." 

It is the largest death toll inflicted among the ranks of the rebels since they started their offensive backed by the Turkish army and planes against Islamic State near the Turkish-Syrian border on August 24.

The incident prompted the rebels to withdraw from the area they had managed to take control of on Sunday, located 13 kilometres south-west of the strategic border town of al-Rai.

Al-Rai is one of the centres of Turkey's recent ground intervention in support of Syrian rebels against Islamic State and Kurdish-led forces.  

Meanwhile, at least two people were killed and 14 others were wounded in two suicide attacks Monday near the local headquarters of al-Assad's ruling Baath Party in the central Hama province, the observatory said.

Islamic State, via its media outlet Aamaq Agency, said three suicide attackers targeted police and Baath party buildings in the area.

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