The US-led coalition appears to have loosened its rules of engagement in Syria and is putting civilian lives at greater risk, a monitoring group has said after reports that at least 56 people were killed in a single attack this week.

The London-based Airwars group says the coalition has conducted "intense" raids targeting Islamic State militants around the north-eastern town of Manbij, including Tuesday's deadly attack in nearby Tuhkar.

That raid came after what has been described as the "worst week" in two years of the coalition campaign in Syria, with at least 55 civilians reported killed around Manbij between 11 and 18 July.

The US has promised to investigate the civilian deaths – but Airwars has said that many more civilians could die due to an apparent shift in the US-led coalition's policy, and fears the coalition is seriously under-reporting civilian deaths caused by its bombs.

Chris Woods, the director of Airwars, told Middle East Eye on Thursday: “We are seeing very intense air strikes on built-up urban areas where significant numbers of civilians are trapped.

“Unfortunately, that has been shown time and again to be a recipe for high civilian casualties. And that’s exactly what we’re seeing around Manbij.

“But in Manbij the number of these events is far higher than what we’ve seen - that concerns us because we think the coalition may have loosened the parameters for their air strikes, putting civilians at greater risk."

And the group said it believes evidence gathered from witnesses, local activists and press reports suggests that the coalition's official death tolls are often hugely under-reported.

Airwars believes up to 190 civilians have died around Manbij since the coalition and its Kurdish-Arab allies in the Syrian Democratic Forces launched an offensive against IS in late May.

The coalition has significantly curbed its activities around the town since Tuesday's strike in Tuhkar. Reported strikes fell from 18 between Monday and Tuesday to just five between Tuesday and Wednesday.

“This appears to be an immediate response [by the US], and is certainly welcome from the perspective of civilians,” Woods said.

The SDF confirmed this week that it had provided intelligence for a coalition strike on Tuhkar on Tuesday, but has denied that any civilians were killed in the bombing.

That is contradicted by local activists who say relatives were struggling to find safe places to bury victims, with some forced to use mass graves.

"The bodies of 73 people have been identified by name," activist Abu Yaman al-Halbi told London-based website al-Araby al-Jadeed, "but around 50 people are so far unidentifiable because their bodies are so badly damaged."

Airwars also says it has evidence from multiple witnesses, as well as several civilians who were injured in the air strike, confirming that the attack was launched by planes from the US-led coalition.

US promises investigation

The US has said it is “aware” of reports of civilian casualties in Syria this week, and has promised a “transparent” investigation.

"We’re aware of reports of civilian casualties,” Defence Secretary Ash Carter said on Wednesday.

“We’ll investigate these reports and continue to do all we can to protect civilians,” he continued. “Being transparent about this issue is a reflection of the civilized nature of this coalition.”

However, Airwars' Woods says he believes that the US has dragged its feet, and under-reports civilian deaths caused by coalition attacks by up to 95 percent.

The US says it has killed a total of 41 civilians in 25 air raids since its bombing campaign began – but to date documents have been released relating to just four of those incidents.

Airwars said it took on average six months between an attack and any public admission by commanders - meaning there would probably be no confirmation of Tukhar or Manbij until well into next year, according to Airwar's Woods.

A spokesperson for Inherent Resolve, the coalition's Syrian operation, refused to speculate as to how long an investigation into Tuesday's strike would take, telling MEE: "The current environment on the ground in Syria makes investigating allegations extremely challenging."

The coalition says it has carried out 450 air raids around Manbij since the SDF imposed a siege on the town two months ago. The US currently counts an air strike as any raid in which one or more munitions are dropped.

UN data from Afghanistan says that every eleventh airstrike there results in a civilian death, while official White House data from operations in countries such as Yemen and Pakistan shows that, on average, one civilian is killed for every seven raids.

Based on the number of strikes launched by the US-led coalition in Iraq and Syria, the 1,500 figure is “absolutely in line with expectations”.

“The coalition claims to have killed just 41 civilians – we think that figure is hopelessly unrealistic.”

A coalitoin spokesperson told MEE: "Coalition airstrikes are the most precise in the history of warfare. Mitigating civilian casualties is a key component of the air campaign and is why we use precision weapons.

"We are continuing to apply the same rigorous procedures to our clearing of fires that we have been.

"We take all reports of non-combatant casualties seriously and assess all incidents as thoroughly as possible. It's important to note that the current environment in Syria makes investigating allegations extremely challenging.

"Until these investigations and associated processes are completed, it is inappropriate to discuss details.

"We have seen [IS] using more civilians as human shields in the Manbij area. We've seen them during the Manbij fight pushing civilians toward the lines of [allied forces] to try to draw fire.

"While the investigative process will provide details on this particular incident, and we don't know what happened, we won't be surprised if this is somehow a factor."