More than 400 civilians were killed by the US-led coalition in air strikes targeting Islamic militants inside two Syrian provinces over the past month, a monitor said on Friday.

This month's death count of 427 from US-led coalition air strikes is more than double the previous 30-day toll.

The Syrian Observatory for Human Rights said the period between 23 May and 23 June saw the highest civilian death toll in coalition raids for a single month since they began on 23 September 2014.

Observatory head Rami Abdel Rahman said 222 civilians, including 84 children, were killed in the largely Islamic State group-held province of Deir Ezzor.

Another 250 civilians, including 53 children, were killed in Raqqa province, where US-backed forces are trying to oust IS militants from their bastion Raqqa city.

He told AFP that the new deaths brought the overall civilian toll from the coalition's campaign to 1,953, including 456 children and 333 women.

Chris Woods, director of Airwars, a group that monitors US-led coalition air strikes in Syria and Iraq, told Middle East Eye that the observatory's findings chime with figures released by other local and national monitoring groups.

"Military and governments involved in the campaign to defeat ISIL need to urgently respond to this deteriorating situation," said Woods.

"Every major NGO and international agency have raised concerns about the high casualties in Raqqa and Mosul," Woods told MEE.

"These latest numbers from the observatory chime with what local and national monitors have been saying for some time, which is that significant numbers of civilians are being killed daily in US and SDF action to capture Raqqa."

'Staggering loss of civilian life'

Earlier last week, United Nations war crimes investigators said that intensified US-led coalition air strikes on Islamic State's stronghold of Raqqa in Syria are causing a "staggering loss of civilian life".

Paulo Pinheiro, chairman of the UN Commission of Inquiry, said that intensified coalition air strikes have killed at least 300 civilians in the city of Raqqa since March.

He also told the Human Rights Council that 10 agreements between the Syrian government and armed groups to evacuate fighters and civilians from besieged areas, including eastern Aleppo, "in some cases amount to war crimes" as civilians had "no choice".

He said: "We note in particular that the intensification of air strikes, which have paved the ground for an SDF advance in Raqqa, has resulted not only in staggering loss of civilian life, but has also led to 160,000 civilians fleeing their homes and becoming internally displaced."

The previous deadliest 30-day period was between 23 April and 23 May during which 225 civilians were killed.

US-led coalition warplanes are supporting an alliance of Kurdish and Arab fighters who are battling to take the city from Islamic State militants.

The Syrian Democratic Forces (SDF) began to attack Raqqa a week ago with the aim of taking it from the militant group. The SDF, supported by heavy coalition air strikes, have taken territory to the west, east and north of the city.

The coalition insists it takes every measure to avoid hitting civilians, including by aborting missile strikes at the last moment if a civilian unexpectedly wanders into the target zone.

A total of more than 320,000 people have been killed since Syria's conflict erupted in 2011 with anti-government protests, according to the Britain-based Observatory.