Eight civilians were killed and three others injured in air strikes carried out from April to July of last year against Islamic State targets in Iraq and Syria, the US military said Friday.

US Central Command (CENTCOM), which oversees operations in the Middle East, said the deaths came during three strikes in Syria and two in Iraq, bringing to 14 the total number of civilians the US-led coalition has acknowledged killing in the two countries since operations against the Islamic State group began in August 2014.

"We deeply regret the unintentional loss of life and injuries resulting from those air strikes," CENTCOM said in a statement revealing the results of their investigations.

Two of the victims died on April 12 near Hawijah in Iraq during a strike on an IS "tactical unit," CENTCOM said, and three others were killed in another strike near Suluk in Syria on June 11.

On July 4, near the IS stronghold of Raqa in Syria, three civilians were "likely" killed during a strike against a high-value IS target. The car and motorcycle they were traveling in had crossed into the target zone after the bomb was released, CENTCOM said.

Though the military used the term "likely," their statement also says that in all the strikes, "civilian casualties unfortunately did occur."

Officials said they had determined the strikes complied with the law of armed conflict, "and all appropriate precautions were taken."

The latest announcement marks the third time since the US-led coalition started bombing IS jihadists that the military has acknowledged civilian casualties.

In November last year, the Pentagon said four people had been killed in a US air strike against an IS checkpoint in Hatra in Iraq in March.

The military has also acknowledged the deaths of two children in Syria in November 2014.

Despite such incidents, US officials frequently boast of the accuracy of their missile strikes and bomb drops.

As of January 10, the coalition had carried out 9,560 strikes, and the Pentagon has said more than 97 percent of such attacks hit their proper targets.

But air campaign critics claim coalition strikes are killing many more civilians than the United States has acknowledged.

Airwars, a London-based group of independent journalists, published a report in August saying "many hundreds" of civilians had been killed.

- Warning IS oil truck drivers -

Still, the United States has gone to considerable lengths to avoid many civilian deaths. In a move ridiculed by hawkish opponents in the US Congress and privately by some coalition partners, pilots dropped pamphlets before bombing trucks ferrying illicit oil around Syria for the IS group.

The military reasoned the truck drivers, while working for the jihadists, were not themselves members of the group.

The Pentagon has said it is prepared to alter rules of engagement when necessary to hit high-value targets, provided these attacks yield significant gains against the jihadists.

A possible illustration of this came Monday, when a coalition strike on an IS facility hoarding millions of dollars in cash reportedly killed five to seven civilians.

CENTCOM spokesman Colonel Pat Ryder told AFP the coalition had received 120 allegations of civilian killings.

Fourteen of these remain under investigation, 19 are closed and the rest were "deemed to be not credible," he said.

"In accordance with our commitment to transparency, we are working to release the assessment findings of the remaining closed allegations as soon as possible," he said.