Shiite Iraqi fighters mobilize May 19, 2015, near the village of Nukhayb in embattled Anbar province. (Mohammed Sawaf/AFP/Getty Images)

American airstrikes have killed at least two civilians since they began in Iraq and Syria last year, the Pentagon said Thursday in its first acknowledgment of noncombatant deaths in the U.S.-led campaign against the Islamic State.

The U.S. Central Command, announcing the results of a four-month investigation, said it had concluded that two children were probably killed in the strikes Nov. 5 and 6 around the city of Harem in Syria. The strikes targeted an al-Qaeda cell known as the Khorasan group.

Centcom is investigating three other reports of civilian casualties: two in Iraq and another in Syria, officials said.

A U.S. defense official, speaking on the condition of anonymity to discuss the Centcom findings, said investigators faced a difficult task in establishing the results of air attacks in a conflict with few U.S. troops on the ground. About 3,000 U.S. troops are training and advising local troops in Iraq; there are no U.S. forces in Syria.

Since August, the United States and allied nations have launched 2,458 airstrikes in Iraq and 1,593 strikes in Syria, mostly against the Islamic State but occasionally against other militants. While coalition aircraft have tried to avoid strikes on urban areas, they have at times dropped bombs inside cities.

At a Senate Armed Services Committee hearing, a panel including retired Gen. John Keane, former vice chief of staff of the Army, told Sen. Lindsey O. Graham (R-S.C.) that the U.S. needs a better strategy to fight Islamic State militants. (AP)

The official said the low civilian death toll that Centcom has established underscored the precision of U.S. airstrikes. “We take all possible measures to prevent casualties to noncombatants,” he said. Human rights groups have claimed much higher death tolls among civilians from some U.S airstrikes.

In a report released in November, the Syrian Network for Human Rights said one of the children killed was a 5-year-old girl named Daniya Ali al-Haj Qaddour, the daughter of a fighter from the Islamist group Jabhat al-Nusra. Video footage that the group said was filmed in the aftermath of the early-November strikes showed destroyed buildings and a vast field of rubble. The group also posted a photo of a young girl in pigtails it said was Daniya, and a link to a photo of what it said was her body.

Centcom said it was not aware at the time of the strikes that any children lived at those sites.

The U.S. announcement of civilian deaths came the same day that the Pentagon said it would expedite delivery of 2,000 antitank weapons to Iraq to help security forces there combat the type of vehicle-borne suicide bombs that helped Islamic State fighters take control of Ramadi over the weekend.

Col. Steve Warren, a Pentagon spokesman, said the AT-4 shoulder-fired antitank weapons are expected to arrive in Iraq as early as next week.

The number of weapons is double what was previewed just one day ago at the State Department, with the Obama administration scrambling to adjust its Iraq strategy in the wake of what it has characterized as a significant setback in efforts to push back the militants. A State Department official had said the arms would be delivered next month.

Officials said the Obama administration had originally planned to provide 1,000 AT-4s but increased that number in an effort to better equip Iraqi forces.

President Obama, in an interview published Thursday, said that the United States has to “ramp up not just training, but also commitment” to Iraqi security forces and Sunni tribesmen in Anbar province, where Ramadi is the capital.

The Ramadi defeat, Obama told the Atlantic, “is indicative that the training of Iraqi security forces, the fortifications, the command-and-control systems are not happening fast enough in Anbar, in the Sunni parts of the country.”

Anbar is Iraq’s largest province, occupying much of the western part of the country.

“We better get Sunni tribes more activated than they currently have been,” Obama said. “So it is a source of concern. We’re eight months into what we’ve always anticipated to be a multi-year campaign, and I think Prime Minister [Haidar al-]Abadi recognizes many of these problems, but they’re going to have to be addressed.”

It’s unclear whether any of the new antitank weapons will go to Sunni tribesmen, who have complained they lack weapons to fight the Islamic State.

Decisions to move rapidly in support of the Sunni tribes follow a months-long internal debate in the administration over whether U.S. attention should be focused on Anbar, as the State Department has advocated, or on the northern city of Mosul, where the Islamic State offensive began last summer.

In his first direct response to the latest round of Republican charges that his move to withdraw U.S. troops from Iraq in 2011 — under a 2008 agreement signed by President George W. Bush — is responsible for the current chaos there, Obama said, “I think it was a mistake for us to go in in the first place, despite incredible efforts that were made by our men and women in uniform.”

Obama said direct responsibility for Iraq’s current situation lies with former prime minister Nouri al-Maliki, whose Shiite-led government was unwilling “to reach out effectively” to Iraq’s Sunni and Kurdish populations.

The administration is also discussing how to effectively use airstrikes, more difficult in an urban environment, as part of that effort.

Coalition aircraft conducted two airstrikes near Ramadi on Thursday.