WASHINGTON -- U.S. airstrikes aimed at Islamic State targets in Syria and Iraq killed 20 civilians between September and February, including eight in a single attack on a mortar position used by militants, the U.S. military said Friday.

The 20 deaths nearly doubled the total number of acknowledged civilian deaths since the U.S.-led air campaign began in 2014. The U.S. has conducted nearly 12,000 airstrikes since bombing began in Iraq in August 2014 and in Syria a month later. The air campaign has been a central element of the U.S. strategy for disrupting and eventually defeating IS.

In announcing the results of several investigations stemming from allegations of civilian casualties, U.S. Central Command said it concluded "the preponderance of evidence" indicates 20 civilians were killed and 11 others wounded in nine attacks between Sept. 10, 2015, and Feb. 2, 2016. All were judged to have been the unintended result of attacks on legitimate targets.

Smoke rises from Islamic State positions following a U.S.-led coalition airstrike during a military operation launched by Iraq Anti-terrorism forces to regain control the western suburbs of Ramadi, the capital of Iraq's Anbar province, 70 miles (115 kilometers) west of Baghdad, Iraq, Saturday, Nov. 14, 2015. AP

Six of the strikes were in Iraq and three were in Syria. Armed drones were involved in some cases; manned aircraft in others.

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Defense Secretary Ash Carter was in Baghdad last week to talk to Iraqi leaders about beefing up Iraqi forces working to retake the northern city of Mosul, a critical goal in the effort to defeat the Islamic State in Iraq and Syria (ISIS). Carter said the U.S. will send 200 more troops and a number of Apache helicopters to Iraq to assist in the figh.

A spokesman for the command, Col. Patrick Ryder, told reporters that the U.S. regrets the casualties and contends that the campaign is the "the most precise air campaign in the history of warfare." He said the military takes great care to avoid civilian casualties.

"There is no such thing as an intentional civilian casualty," Ryder said. "We are attempting to avoid civilian casualties, but in these cases, unfortunately, we assess that it was likely that civilians had died." He added, "In this type of armed conflict, particularly with an enemy who hides among the civilian population, there are going to be, unfortunately, civilian casualties at times."

Since the start of U.S. airstrikes in 2014, the U.S. has acknowledged a total of 41 civilian deaths in Iraq and Syria, including those announced Friday.

The most deadly of the nine investigated airstrikes, in terms of civilian casualties, was an Oct. 5, 2015, attack on an Islamic State mortar position in Atshanah, Iraq. Central Command said eight civilians were killed; it did not elaborate on the circumstances.

On Dec. 12, 2015, a strike on a suspected Islamic State checkpoint in Ramadi killed five civilians. Central Command said they "unexpectedly moved into the target locations after weapons were already in flight."