The Pentagon says the US-led coalition airstrikes against purported Daesh targets in Syria and Iraq have killed over 600 civilians since 2014.

The US Department of Defense said in a report on Friday that a total of 603 civilian lives had been lost between August 2014 and May 2017, with nearly half of all those deaths occurring in or near the northern Iraqi city of Mosul, the last remaining stronghold of the Takfiri terrorist group in the Arab country.

The Pentagon's estimate of civilian deaths comes as independent monitors have time and again challenged such reports and revealed that the US-led military campaign has significantly inflicted more civilian casualties.

Airwars, a UK-based non-profit monitoring group, says at least 4,354 civilians have been killed by coalition air strikes in Iraq and Syria during the same period.

The co-called Syrian Observatory for Human Rights says the 42,234 documented airstrikes in the country have resulted in a minimum estimate of some 7,000 civilians killed by the US-led coalition between 2014 and 2017.

In March, Amnesty International accused the coalition of "flagrant violation of international humanitarian law" as it failed to take precautions to prevent a high death toll among civilians in the key Iraqi city of Mosul.

A French Rafale warplane lands on the deck of France's aircraft carrier Charles-de-Gaulle operating in the eastern Mediterranean Sea on December 9, 2016, as part of a US-led coalition against the Daesh Takfiri group. (Photo by AFP)

The United States launched a campaign of airstrikes against Daesh in August 2014 after the terrorist group overran Mosul and parts of Iraq's north and west.

The US-led coalition of 68 nations has been conducting airstrikes against what are said to be the positions of Daesh terrorists inside Syria since September 2014 without any authorization from the Damascus government or a UN mandate.

The coalition has repeatedly been accused of targeting and killing civilians. It has also been largely incapable of fulfilling its declared aim of destroying Daesh.