More civilians have died in US-led air and artillery strikes in Iraq and Syria, officials acknowledged Thursday amid criticism the American military is being less cautious about killing innocents.

The US-led coalition said that in October, an ongoing review of civilian casualty reports showed at least 801 civilians had been unintentionally killed since operations to defeat the Islamic State group in Iraq and Syria began in late 2014.

The toll is up 15 from 786, when the coalition in late October last announced its findings.

As operations against IS wind down, the number of new strikes has shrunk dramatically, and on Thursday the Pentagon said 400 Marines who had been supporting local forces in the operation to recapture Raqa in Syria were coming home.

But critics say the US-led coalition has been taking greater risks with civilian lives since President Donald Trump took office and gave the military far greater leeway in how it conducts strikes compared to when Barack Obama was in power.

Monitoring group Airwars insists the official count is well below the true civilian toll of the bombing campaign, estimating that almost 6,000 innocent lives have been lost.

Military officials insist the rules of engagement are unchanged, but under Obama many more strike decisions involved civilian approval.

"Clearly the Trump administration has opened the aperture on what is acceptable risk, whether it's a special operations attack or a bombing raid, they are more aggressive," said Congressman Adam Smith, who is the top Democrat on the House Armed Services Committee providing military oversight.

- 'Constantly improving' -

Numbers reflect this shift, with US forces in Afghanistan on course to triple the number of bombs dropped this year compared to last.

In Africa, drone strikes and ground operations against jihadists have increased dramatically.

A detailed, on-the-ground investigation by The New York Times in Iraq found one in five US-led strikes resulted in civilian casualties in that country, wildly different from the coalition claim that only 0.35 percent of strikes result in "credible" claims of civilian casualties.

The report described the opaque and often-impossible task civilians who survived strikes, or the families of those killed, have in trying to get compensation from the coalition.

The coalition appears aware of the criticisms.

In its statement, it said it is "constantly improving its visibility on and accountability for allegations" and noted that it had trained about 30 people to help weigh claims.

"We continue to hold ourselves accountable for actions that may have caused unintentional injury or death to civilians," the statement read.

In one strike detailed by the coalition on Thursday, five civilians were killed March 3 near Tabqah in Syria during a strike on an IS "weapon storage facility."

Four more had been killed in the same area on February 4 during a strike on an IS "headquarters."