A Russian air strike has killed at least 39 people at a prison run by an al-Qaeda affiliate group in Syria, activists have said.

The London-based Syrian Observatory for Human Rights has said at least 39 people have been killed but another group - the Local Coordination Committees - has said the death toll is at 51.

Agence France Presse has reported that five civilians were among the dead - including a child.

The Observatory said four missiles hit an Islamic Court attached the jail as well as a nearby road that connected the court with a nearby market.

Others killed included many fighters from the al-Nusra Front as well as detainees of the prison in the northwestern town of Maarat al-Numan.

The al-Nusra Front is one of the many non-Isis jihadist groups operating in Syria which are being targeted by Russian air strikes.

British jets prepare for air strikes in Syria Show all 10 1 /10 British jets prepare for air strikes in Syria British jets prepare for air strikes in Syria A Tornado jet takes off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria British jets prepare for air strikes in Syria Pilots and ground crew prepare combat aircraft Panavia Tornados at RAF Marham at RAF Marham, UK Getty British jets prepare for air strikes in Syria A Eurofighter Typhoon jet takes off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria British jets prepare for air strikes in Syria A RAF Tornado arrives at RAF Akrotiri to begin operations in Akrotiri British jets prepare for air strikes in Syria A Tornado jet ahead of taking off from RAF Lossiemouth in Scotland, as RAF Tornado jets carried out the first British bombing runs over Syria, the Ministry of Defence has confirmed. The air strikes were carried out within hours of a vote by MPs in the Commons to back extending operations against Isis from neighbouring Iraq British jets prepare for air strikes in Syria Personnel work on a British Tornado after it returned from a mission at RAF Akrotiri in southern Cyprus British jets prepare for air strikes in Syria Two RAF Tornado GR4's, both with remaining weapons ordnance, approach RAF Akrotiri in Cyprus, as they return to the base after carrying out some of the first British bombing runs over Syria British jets prepare for air strikes in Syria A RAF Tornado takes off from RAF Akrotiri, on the Mediterranean island nation of Cyprus British jets prepare for air strikes in Syria A Tornado jet leaving RAF Lossiemouth in Scotland British jets prepare for air strikes in Syria AKA RAF Tornado arrives at RAF Akrotiri to begin operations in Akrotiri, Cyprus. The RAF has sent two further Tornado aircraft and six Typhoons to bolster aircraft now flying sorties to both Iraq and Syria

Several opposition groups control territory in the north west corner of Syria - including Idlib and parts of Aleppo - but Syrian government forces are advancing in part due to the intervention of Russia.

Around 70 per cent of the 5,000 air strikes reportedly carried out by Russia since they launched their offensive in September last year have hit Isis controlled targets, an US official told reporters.

Human rights groups and aid agencies have also accused Russia of killing civilians in markets and residential areas during its "imprecised" campaigns - this is a charge Russia has denied.

UK forces began air strikes against the country at the end of last year but have so far focused on targeting Isis controlled areas.

Isis and al Nusra are bitter rivals despite both being hardline Islamist rebels fighting against the Assad government.