Washington: An American bombardment that killed an Islamic State group cyber-operative in Syria last August also killed three civilians, the US Army acknowledged on Friday.

The bombing August 24 in the IS stronghold of Raqqa, which killed cyber-combatant Junaid Hussain, was one of four incidents alleged to have caused civilian casualties that the US Central Command said it had closely analyzed.

"It was assessed three civilians in the vicinity of the strikes were killed and five more were injured," said the Central Command, which is in charge of American forces in the Middle East and North Africa.

The statement also acknowledged the death of two other civilians in a coalition air strike on September 24 near Sinjar, in northern Iraq, as well as injuries to a total of three civilians in two other attacks.

The five civilians killed would bring to 21 the number of likely civilian victims officially recognized by the coalition since it began launching air strikes against Islamic State militants in August 2014.

"We deeply regret the unintentional loss of life and injuries resulting from those strikes," the Centcom statement said.

The coalition generally acknowledges the civilian victims of its strikes only after lengthy investigations. It has been accused of underestimating their numbers.

Airwars, an NGO based in London, which compares and compiles publicly available information, estimates that actual civilian deaths could number in the hundreds, possibly even exceeding 1,000.

According to some sources, the coalition has begun accepting a greater risk of civilian casualties when bombing attacks look likely to give the coalition a significant advantage.

Junaid Hussain, who was killed in the August attack, used Twitter and social networks to incite IS sympathizers around the world to launch "lone wolf" style attacks.

American officials say he was notably implicated last spring in compiling and circulating a list of 100 American military figures to be killed.

The coalition has launched some 9,800 air strikes since the start of its campaign against the IS jihadists.