The US-led coalition bombing the Islamic State group in Syria and Iraq said Thursday that 54 civilians had been "inadvertently killed" in seven air strikes between March and October.

The announcement brings the official coalition tally of civilians killed to 173 since the anti-IS campaign began in the fall of 2014 -- though critics say the real figure is far higher.

"Although the coalition makes extraordinary efforts to strike military targets in a manner that minimizes the risk of civilian casualties, in some cases casualties are unavoidable," the coalition said in a statement.

The deadliest strike occurred July 18, when coalition aircraft attacked a group of IS fighters near Manbij in Syria, killing about 100 of them.

But "up to 24 civilians who had been interspersed with combatants were inadvertently killed in a known (IS) staging area where no civilians had been seen in the 24 hours prior to the attack," said the coalition.

In a July 28 strike, also in Syria, 15 civilians were killed when a moving IS vehicle that had been targeted slowed in a populated area.

Officials said the vehicle had slowed after the guided bomb was released.

On October 22, a strike in Iraq on an IS position being used to attack Iraqi forces killed eight civilians.

Pentagon spokesman Navy Captain Jeff Davis said Thursday's announcement marked the start of a "more regular process" of releasing civilian-casualty information.

"We, as you know, go to extraordinary lengths to protect against civilian casualties in the first place," he told reporters.

"No other nation (or) coalition in the history of warfare does what we do to exercise extraordinary care in carrying out air strikes to prevent them."

Airwars, a London-based collective of journalists and researchers, uses local sources, photographs and media accounts to keep a detailed list of every known coalition air strike.

They have praised Pentagon efforts at accountability compared with other actors in Syria such as Russia and the regime of President Bashar al-Assad, but the group says the number of likely civilian deaths from coalition strikes is 1,915 at a bare minimum.

As of November 17 -- the date of the most recent tally -- the coalition had conducted a total of 16,291 strikes, about two-thirds of them in Iraq and the rest in Syria.

Coalition officials said they had also recently reviewed 12 other reports of civilian casualties but these were deemed to be "non-credible."