The long-awaited coalition program to train the Syrian moderate opposition could begin by early spring, and officials are beginning to identify individual fighters who could participate, the Pentagon said Tuesday.

Navy Rear Adm. John Kirby, Pentagon press secretary, also said for the first time that the U.S. military is conducting several investigations into reported civilian casualties that may have occurred in the U.S.-led coalition airstrikes against Islamic State militants. He said he had no additional information on the incidents, but military leaders said earlier that they were unaware of any civilian deaths resulting from the bombings.

The effort to train Syrian rebels is part of the overall campaign to defeat the Islamic State militants who seized large swaths of Syria and Iraq. Kirby said that once the training program gets up and running it may require additional U.S. forces to assist with the instruction. He did not provide details on how many. Turkey, Qatar and Saudi Arabia have agreed to provide training locations.

The U.S. is involved in a plan to train both moderate Syria rebels and to train and assist Iraqi security forces in order to beat back the Islamic State militants' rampage in both countries.

Kirby said IS momentum in Iraq has stalled and IS fighters have largely been in a defensive posture for several weeks.

"Whatever momentum they had been enjoying has been halted, has been blunted. That has stayed steady over the last couple of weeks," Kirby said. "It's very much a contested environment, but what we don't see, what we haven't seen in the last several weeks has been any renewed offensive moves."

He added that there are still a number of areas controlled or threatened by the IS group, including Mosul and parts of Anbar province in Iraq and portions of northern Syria, including Kobani.

U.S. forces have been flowing into Iraq to conduct training. About 170 troops from the 1st Infantry Division began training Iraqi forces in late December at Taji, north of Baghdad. And there are about 320 U.S. troops at al-Asad Air Base in western Iraq, training members of the Iraqi Army's 7th Division. Those U.S. troops are mostly Marines and they started the training on Dec. 20.

As of this week, there are about 2,140 U.S. troops in Iraq to train, advise and assist the Iraqis and provide security for the American forces. President Barack Obama has authorized the deployment of a total of 3,000 U.S. troops to Iraq, and Kirby said the remainder of that force should arrive in the country in the next four to six weeks.

On the civilian casualties, Kirby referred reporters to Central Command for details such as the number of alleged deaths and when they came to Central Command's attention. Officials at Central Command said they were unable to immediately provide answers but were preparing a statement with further details.

As recently as last month, Lt. Gen. James L. Terry, the top commander of the coalition that is fighting the Islamic State in Iraq and Syria, told reporters at a Pentagon news conference that his organization is extremely careful and deliberate about its targeting.

"We have some great capability in terms of precision," Terry said, speaking of airstrikes in Iraq and efforts to avoid hitting either civilians or friendly Iraqi forces. "To date, we've got a very good record. I am tracking no civilian casualties."