At least 21 civilians have reportedly died fleeing intense US-led bombing and artillery fire as the battle for the capital of Isis’s so-called caliphate finally begins.

The Syrian Democratic Forces (SDF) Kurdish-Arab coalition announced on Tuesday that an assault on Raqqa from the north, east and west began on Monday, and troops had already managed to enter the city’s eastern gate.

Families trying to escape the all-out attack were killed on Monday night on boats on the Euphrates River, both Syrian state media and opposition war monitor the Syrian Observatory for Human Rights said. Women and children were reportedly among the dead.

Unverified reports from local sources said another 12 were killed in strikes last Friday.

Women secretly capture life inside Raqqa

While Friday’s deaths were not definitively confirmed by either US Central Command or sources on the ground, they would not be out of the ordinary. AirWars, a non-profit monitor which tracks the international coalition’s war against Isis, said there has been a 400 per cent increase in civilian casualties caused by US-led bombing since the twin assaults on Raqqa and Mosul in neighbouring Iraq got underway last autumn.

The battle for Isis’s last stronghold of Raqqa – designed to dovetail with the final stages of the campaign for Mosul – is expected to be long and bloody.

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

The Kurdish-Arab coalition of ground fighters has been slowly tightening the siege around jihadi militants in the northern city since November, assisted by coalition air strikes.

Those strikes have intensified in recent weeks – and caused more and more civilian deaths in the process. According to data from AirWars, at least 220 civilians are estimated to have been killed in May alone.

Western coalition bombing of Isis targets is conducted without the consent of the Syrian government, with which the US does not have official diplomatic ties, and has long been criticised by human rights groups for causing unnecessary loss of life.

Central Command’s own tally of civilian casualties is usually far lower than estimates made by NGOs and monitors on the ground.

“We have been expressing concern for some months already at the steeply escalating death toll before the assault proper began yesterday,” Chris Woods, AirWar’s director, told The Independent.

“We would not have expected this six months ago. All we can do, going on the data we have, is assume that some protections for civilians may have been lifted for the Raqqa assault.”

US President Donald Trump promised on the election campaign trail to to focus his foreign policy on eliminating the group, and his administration has given the Pentagon greater executive control over anti-Isis military operations.

Defence Secretary James Mattis said last week that the US is “accelerating the tempo” of the fight against Isis and the battle has shifted from attrition to “annihilation tactics”.

While US forces operating in Syria and Iraq do “do everything humanly possible” to prevent the loss of innocent life, “civilian casualties are a fact of life in this sort of situation,” he added.

Faced with a fight to the death with no prospect of surrender, Isis are more likely than ever to use Raqqa’s 220,000 civilians as human shields.

“We’ve already seen in Mosul the coalition using very high concentrations of firepower on densely populated areas,” Mr Woods continued. “The concerns we have for Raqqa are even greater.”

Unlike in the battle for Mosul, the US and SDF forces are not local, do not know the area, and are not working with a cooperative government.

The ensuing lack of access for humanitarian agencies and international media means that both ground and air force actions will not be subject to the same level of scrutiny as the US-backed Iraqi coalition forces.

“There has been a worrying number of civilians killed from coalition airstrikes across Raqqa in recent months and the UN reported schools and health clinics in the city were hit by air strikes in April.

“Given the large number of people still trapped in Raqqa it is essential the upmost care is taken to prevent further civilian casualties,” Thomas Garofalo, the International Rescue Committee's Middle East director of public affairs, said in a statement.

Up to half of Raqqa’s population is expected to flee the fighting in coming months, facing Isis snipers and improvised explosive devices as well as the threat of Western bombing. Coalition leaflets dropped on the city have urged civilians to leave their homes - but war monitors and NGOs are calling on US-backed forces to do more to ensure residents’ safety.