A London-based monitor says US airstrikes on two Syrian provinces killed 472 civilians last month, more than double a previous 30-day toll.

The so-called Syrian Observatory for Human Rights said on Friday the period between May 23 and June 23 saw the highest civilian death toll in US airstrikes since they began in September 2014.

Observatory head Rami Abdel Rahman said 222 civilians, including 84 children, were killed in the Dayr al-Zawr. Another 250 civilians, including 53 children, were killed in Raqqah province, he added.

The new deaths brought the overall civilian toll from the US-led aerial attacks to 1,953, including 456 children and 333 women, the French news agency quoted him as saying.

The previous deadliest 30-day period was between April 23 and May 23 this year that claimed 225 civilian lives.

The increase has led human rights groups to question whether changes in procedure are responsible after US President Donald Trump shifted more authority over military operations to the Pentagon.

Several former American security officials warned Pentagon chief Jim Mattis recently that civilian casualties “can cause significant strategic setbacks” by reducing local cooperation and providing fuel for Takfiri militants.

American-backed Kurdish militants in Syria have been advancing toward Raqqah with heavy support from US airstrikes since November.

Turkey's defense minister on Friday warned that Ankara would retaliate against any threatening moves by the YPG militia in Syria after US pledge to take back weapons from the group after the defeat of Daesh.

Fikri Isik told broadcaster NTV that a letter sent to him by Mattis regarding the weapons given to the YPG was a "positive step" but "implementation is essential".

Ankara has said supplies to the YPG have in the past ended up in the hands of PKK, which has fought an insurgency in southeast Turkey since the mid-1980s.

The London-based observatory said on Wednesday Turkey had sent reinforcements, including troops, vehicles and equipment into Syria, towards areas south of Azaz town, which is held by Turkish-backed militants.

Turkey sent tanks and warplanes across the border into northern Syria in August last year, carving out a big portion of the area.