WASHINGTON (Reuters) - Six U.S. air strikes against al Qaeda and Islamic State targets in Iraq and Syria killed 14 civilians and injured another between July 28 last year and April 29 this year, the U.S. military said on Thursday.

The strikes took place against Khorasan Group, an al Qaeda offshoot in Syria, and Islamic State facilities and vehicles in Iraq, the military said.

"We deeply regret the unintentional loss of life and injuries resulting from our airstrikes and express our sympathies to those affected," said a statement from U.S. Central Command, which oversees U.S. military operations in the Middle East.

The U.S. military has in the past made similar disclosures of civilian deaths in its air strikes against militant groups in Iraq and Syria, and the disclosures typically follow investigations lasting weeks or months to determine the veracity of reports of civilian deaths.

Thursday's report said that in one instance, three civilians were killed in a strike on July 28, 2015 when their vehicle appeared in the target area after the plane had already fired its weapon, the military said.

In another case on April 29, a U.S. airstrike targeting Neil Prakash, an Islamic State member in Mosul, Iraq, struck and killed three civilians on the road and one civilian on an adjacent compound, the military said.

The U.S. military has started a formal investigation into allegations that U.S.-led coalition air strikes killed civilians on July 19 in Syria, a U.S. military spokesman said on Wednesday.

(Reporting by Yeganeh Torbati; editing by Grant McCool)