“The fighting is raging on as I speak and is expected to last several weeks until the dam, airfield and city are free from ISIS control,” said Col. Joseph E. Scrocca, a spokesman for the American-led command in Baghdad.
As the operation unfolded, Syrian state television and local residents asserted that at least 30 Syrian civilians were killed in an airstrike that hit a school where they had taken shelter in a rural area of Raqqa Province on Tuesday. American military officials acknowledged that the United States had been carrying out airstrikes in the area. These officials said they could not confirm the reports of civilian casualties, but would investigate.
As the battle for Raqqa has accelerated, the number of airstrikes has climbed. Colonel Scrocca said that over the past four months the American-led coalition had conducted more than 300 such strikes around Tabqa and west of Raqqa, and that enemy fighters, fortifications and vehicles had been targeted.
Describing the Tabqa operation, American officials said that a ground force of Syrian fighters was approaching the dam from the north. The airlift was carried out south of the dam, Colonel Scrocca said, and appeared to take the militants by surprise, Colonel Scrocca said.
Important details of the operation, including how many Syrian fighters and American advisers were involved, were not disclosed. News reports suggested 500 Syrian fighters had been deployed, but American officials hinted it could be much more.
“It could be 500; it could be a heck of a lot more,” Colonel Scrocca said in a briefing that was broadcast into the Pentagon.
American artillery and attack helicopters have not previously been employed in Syria. With this Tabqa operation, the American strategy in Syria has come to resemble the operation to retake Mosul, Iraq’s second-largest city, where conventional American military forces have been combined with the use of advisers to support local forces who are doing the main fighting on the ground.
One big difference, however, is the complexity of the local forces with which the United States is aligning itself.
In Iraq, the United States is supporting Iraqi government forces, namely Iraq’s Counterterrorism Service and its Army and Federal Police. In Syria, in contrast, the United States has been working with Arab fighters, the Kurdish Y.P.G. militia and local tribes.
Adding another layer of complexity, Turkey had objected vociferously to the role of the Kurdish Y.P.G. While American military commanders believe the Y.P.G. has some of the most experienced and proficient fighters, Turkey has denounced the organization as a terrorist group.
With Turkey’s concerns in mind, American military officials emphasized that Syrian Arabs made up 75 percent of the fighters in the Tabqa operation — while acknowledging that Syrian Kurds were also involved in the assault.
President Trump, who asserted during his campaign that he had a secret plan to defeat the Islamic State, has yet to outline a new strategy to “demolish and destroy” the militant group, as he told a joint session of Congress last month.
So far, the Trump administration has been operating within the broad contours of the approach by the Obama administration, which called for training and equipping local fighters, supporting them with American firepower and advisers — and, when deemed critically important, sending American commandos to hit high-value targets.
But the Trump White House has dispensed with the detailed and often prolonged review of operations and tactics that were conducted under the Obama administration. The change has allowed American commanders to step up the pace of their operations.
Pentagon officials said that Defense Secretary Jim Mattis was informed of the Tabqa operation, as was the White House, but the assault was being carried out within the authority that has been delegated to American military commanders.
More flexibility for American commanders appears to be coming. Representative Mac Thornberry, Republican of Texas Republican and the chairman of the House Armed Services Committee, told reporters Wednesday that he expected the White House to remove “artificial troop caps” in Syria, Iraq and Afghanistan.
The current “force manning level” for Syria sets a limit on the number of American military personnel in Syria at 503. But the limit does not count temporary reinforcements, like the roughly 400 personnel who were deployed in Syria when the Marine artillery battery and Army Rangers were sent to the country.
There was another telling indication on Wednesday that American Special Operations would continue to play an important role. Col. Jonathan P. Braga, the chief of staff of the Joint Special Operations Command and the former deputy commander of Delta Force, has been named as the next senior operations officer for the American-led command that is leading the campaign against the Islamic State in Iraq and Syria.
With the stepped-up pace of military operations against the Islamic State and Al Qaeda, which is also operating in Syria, there has also been an increase in reports of civilian casualties. In the Tuesday episode, the Syrian Observatory for Human Rights said that at least 33 civilians were killed when a school in Mansoura, a town 15 miles from Raqqa, was struck.
The United States Central Command is investigating an airstrike last week in Al Jinah, a village in western Aleppo Province.
American military officials said they had struck a gathering of Qaeda leaders and operatives and had killed dozens of militants. But residents said the airstrike had struck a mosque that was used by civilians for a weekly religious meeting and that civilians had been killed and wounded.