Air strikes carried out by the US and its coalition partners in Syria have killed the highest number of civilians on record since the bombing campaign began, a war monitor has said.

A total of 225 civilians, including 36 women and 44 children, were killed in the period between 23 April to 23 May, the UK-based Syrian Observatory for Human Rights said.

The toll is the highest number of recorded deaths since the international air campaign against Isis began in September 2014.

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“The past month of operations is the highest civilian toll since the coalition began bombing Syria,” Observatory head Rami Abdel Rahman told AFP news agency.

“There has been a very big escalation.”

At least 122 Isis fighters and eight members of militias loyal to the Syrian government were also killed in the same period, the Observatory said.

The strikes are conducted without the consent of the Syrian government, with which the US does not have official diplomatic ties, and have long been criticised by Damascus and Syria’s allies in Moscow and Tehran for causing unnecessary loss of life.

One incident in 2016 a strike designed to take out Isis weapons depots and other positions near Deir Ez Zour in the north of the country accidentally targeted Syrian army positions instead, killing 62 soldiers.

In Pictures: The crisis unfolding in Syria Show all 30 1 /30 In Pictures: The crisis unfolding in Syria In Pictures: The crisis unfolding in Syria A Syrian family arrives at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods Getty In Pictures: The crisis unfolding in Syria A Syrian woman, fleeing violence in the restive Bustan al-Qasr neighbourhood, reacts as she stands with her children in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past resident fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops, the Syrian Observatory for Human Rights said Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood , after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian pro-regime fighters, gesture as they drive past residents fleeing violence in the restive Bustan al-Qasr neighbourhood, in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood, after regime troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian residents, fleeing violence in the restive Bustan al-Qasr neighbourhood, arrive in Aleppo's Fardos neighbourhood Getty In Pictures: The crisis unfolding in Syria A Syrian pro-regime fighter speaks with a child, as residents flee violence in the restive Bustan al-Qasr neighbourhood. Syrian rebels withdrew from six more neighbourhoods in their one-time bastion of east Aleppo in the face of advancing government troops AFP/Getty Images In Pictures: The crisis unfolding in Syria Smoke rises as seen from a governement-held area of Aleppo, Syria Reuters In Pictures: The crisis unfolding in Syria Syrian soldiers targeting rebels-held areas in the eastern neighborhoods in Aleppo, Syria. According to media reports, the army is now holding on 99 percent of Aleppoís eastern neighborhoods EPA In Pictures: The crisis unfolding in Syria Syrian pro-government forces patrol Aleppo's eastern al-Salihin neighbourhood after troops retook the area from rebel fighters Getty In Pictures: The crisis unfolding in Syria Syrian soldiers rest following the battle at al-Sheik Saeed neighborhood in Aleppo, Syria EPA In Pictures: The crisis unfolding in Syria A Syrian pro-government fighter walking past closed shops in the Bab al-Nasr district of Aleppo's Old City. Once renowned for its bustling souks, grand citadel and historic gates, Aleppo's Old City has been rendered virtually unrecognisable by some of the worst violence of Syria's war Getty In Pictures: The crisis unfolding in Syria The crucial battle for Aleppo entered its 'final phase' after Syrian rebels retreated into a small pocket of their former bastion in the face of new army advances. The retreat leaves opposition fighters confined to just a handful of neighbourhoods in southeast Aleppo, the largest of them Sukkari and Mashhad Getty In Pictures: The crisis unfolding in Syria Syrian civilans arrive at a checkpoint, manned by pro-government forces, at the al-Hawoz street roundabout, after leaving Aleppo's eastern neighbourhoods. Syria's government has retaken at least 85 percent of east Aleppo, which fell to rebels in 2012, since beginning its operation Getty In Pictures: The crisis unfolding in Syria Syrian civilians flee the Sukkari neighbourhood towards safer rebel-held areas in southeastern Aleppo Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances Getty In Pictures: The crisis unfolding in Syria Syrians celebrate in the government-held Mogambo neighbourhood of the northern Syrian city of Aleppo, after rebel fighters retreated into a small pocket of their former bastion in the face of new army advances. The fall of Aleppo would be the worst rebel defeat since Syria's conflict began in 2011, and leave the government in control of the country's five major cities Getty In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee Aliya inside the tent where she lives with her husband and ten children in a camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria Syrian refugee women and children outside the entrance to their tents in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A Syrian refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA Wire In Pictures: The crisis unfolding in Syria A Syrian refugee woman outside the entrance to the tent where her family live, in the refugee camp in the Bekaa Valley, Lebanon, close to the Syrian border PA wire In Pictures: The crisis unfolding in Syria A vehicle drives past a mosque at night in Idlib, Syria. Picture taken with a long exposure Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria The night sky is seen through damaged windows in the rebel-controlled town of Binnish in Idlib province, Syria Reuters In Pictures: The crisis unfolding in Syria Damaged buildings stand in the rebel-controlled area of Maaret al-Numan in Idlib province, Syria Reuters

However, since US President Donald Trump entered office in January this year there has been a marked uptick in civilian deaths in bombing operations against Isis across both Syria and neighbouring Iraq.

In March, the US was accused of killing around 300 civilians alone after one strike which hit a mosque in Aleppo province and two incidents in the fight for Isis-controlled neighbourhoods of the Iraqi city of Mosul.

Earlier this month, the US military said that coalition air strikes in Iraq and Syria had ”unintentionally“ killed 352 civilians since the campaign began, but rights groups have blasted the estimate as too low, saying the US is guilty of not taking “sufficient precautions” to avoid civilian deaths.

SOHR’s own estimate is that 1,481 people, among them 319 children, have been killed by US-led air operations since 2014.

US investigations into the three March incidents are still underway.

The Pentagon has denied there has been any significant change in US-led bombing strategy since former President Barack Obama left office.

On Friday, however, US Defence Secretary Jim Mattis said Mr Trump has “directed a tactical shift from shoving Isis out of safe locations in an attrition fight to surrounding the enemy in their strongholds so we can annihilate [them].

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”The intent is to prevent the return home of escaped foreign fighters,“ he added.

Isis now holds onto just a fraction of the territory under its control at the height of the group’s powers in 2014.

Twin US-backed campaigns to oust fighters from their last urban strongholds - Raqqa in Syria and Mosul in Iraq - are underway, led by local forces on the ground assisted by 5,000 US military advisors.