International concern has grown about the humanitarian toll of the US-backed operation to recapture Raqqa, the de facto capital of the self-styled Islamic State (IS) militant group in Syria.

Earlier this week, UN war crimes investigators said airstrikes launched by the US-led coalition against IS have caused a "staggering loss of civilian life."

"Civilians, who take no part in the fighting, are in the unenviable role of being the target of most warring parties," Paulo Sergio Pinheiro, who chairs the UN Commission of Inquiry on Syria, said in a statement released on Wednesday. The commission investigates war crimes and crimes against humanity committed during Syria's civil war.

Read more: The Middle East's complex Kurdish landscape

'At the expense of civilians'

IS took control of Raqqa in January 2014, the year it seized several territories in both Syria and Iraq. The group has especially taken advantage of instability in Syria, where a six-year conflict involving global superpowers, regional actors and terrorist organizations has left more than 300,000 people dead and half the population displaced.

On June 6, the US-backed Syrian Democratic Forces (SDF), an alliance of Kurdish and Arab fighters, launched a major operation to recapture Raqqa from IS. 

The Kurdish-led SDF, which spent approximately seven months surrounding Raqqa and preparing for the operation, has now recaptured areas in the northern, eastern and western parts of the city.

UN officials have warned of the fallout from the operation, including the likelihood of a mass exodus of civilians fleeing fighting. An estimated 160,000 civilians remain in the city, according to the UN's humanitarian office.

"The imperative to fight terrorism must not, however, be undertaken at the expense of civilians who unwillingly find themselves living in areas where ISIL is present," Pinheiro said in his statement, referring to the group, by a common acronym.

Abdalaziz Alhamza, a co-founder of the activist group Raqqa Is Being Slaughtered Silently, told DW that the United States, as the SDF's main supporter, needs to do more for civilians displaced by the fighting.

US forces must take more precautions when conducting airstrikes in areas where civilians reside, Alhamza said. He also called on US leaders to place increased pressure on the SDF to adhere to civilian protections in order to prevent human rights violations throughout the operation.

"When they started their campaign against ISIS in Syria and Iraq, and especially on Raqqa, they were very careful at the beginning," Alhamza said, using another acronym for IS. "But, recently, the shelling, the airstrikes have randomly targeted the city."

Rules of engagement

Since May, independent monitors have brought attention to an increased number of civilian casualties in the lead up to the Raqqa operation and its subsequent launch.

"May was the second deadliest month for civilians in Iraq and Syria since coalition airstrikes began in August 2014," Airwars, a journalist-led initiative to monitor the conflicts in Syria and Iraq, reported on Thursday. "The month saw record numbers of strikes and of munitions released, leaving those civilians caught between ISIL and the coalition in even worse straits."

Airwars noted that, since February, there has been little correlation between the number of strikes conducted and civilian casualties.

"This may indicate that the unprecedented increase in fatalities from coalition actions is related to an undisclosed change in the rules of engagement or offensive procedures on the battlefield," Airwars reported.

Read more: US plan to 'annihilate IS' raises questions over civilian toll, larger strategy

Last week, media reports surfaced that the US military had doubled the size of its team investigating civilian casualties, a move cautiously welcomed by groups advocating protections for civilians affected by conflict.

"Investigating reports of civilian harm from coalition activities is a good start, but it's far better to avoid harming civilians if at all possible in the first place," said Federico Borello, executive director for the Washington-based Center for Civilians in Conflict.

"The US must protect civilians in its operations and show the world it values human life even while fighting those who do not," Borello said.

An evolving dilemma

Coinciding with increased migration to urban centers, densely populated cities in the Middle East and elsewhere have often become the scenes of high civilian casualties when they become theaters of war.

"As the world urbanizes, so does conflict," the International Committee of the Red Cross City reported recently. "City centers and residential areas are now the battlefields and front lines of our century."

"There, armed conflicts are waged with weapons designed for use on open battlefields, amplifying their destructive power in the crowded city," the Red Cross reported.

Read more: Mosul: the last stand for 'Islamic State' in Iraq

To imagine that the death and destruction that accompanied the operation to recapture Mosul in Iraq, as well as the monthslong siege of Aleppo in Syria, will not similarly be witnessed in Raqqa is to deny the features of urban warfare in the 21st century."Ultimately, the only way to end civilian suffering is to end this war," Pinheiro said, alluding to a feat that remains elusive on all fronts of the conflict.