US says Syria-Iraq civilian deaths from airstrikes in 'hundreds'

US-led coalition airstrikes on Islamic State group territories in Syria and Iraq have killed 484 civilians since mid-2014, the US military claimed on Friday.

It comes after an airstrike by Operation Inherent Resolve coalition aircraft bombed an IS sniper position in west Mosul in March killed 105 civilians when explosives collapsed a building on top of civilians.

There have been other claims of mass casualties from other airstrikes in Syria and Iraq.

The Inherent Resolve task force insists it does everything possible to avoid civilian casualties and publishes all. 

It stressed only 0.27 percent of its 21,035 strikes since the operation began had produced "credible" civilian death reports.

The US claims are seriously disputed by monitoring groups and non-governmental organisations who say the coalition under-reports civilian deaths.

Airwars, a journalist collective based in London that compiles data from public sources, estimated more than 3,800 non-combatants were killed since the operations began in August 2014.

The Syrian Observatory for Human Rights estimates that in the month from April 23 to May 23, 225 civilians were killed in Syria alone, the heaviest monthly toll since 2014.

The official report Friday only covers reported incidents through the end of April.