But one option being considered is for American troops to step up their support of the fighters by firing artillery, shooting mortars, helping with logistics and significantly expanding efforts to advise them, much as the United States is doing for Iraqi forces in the battle for Mosul.
In late January, President Trump gave the defense secretary, Jim Mattis, 30 days to develop a “preliminary plan” to defeat the Islamic State, also known as ISIS or ISIL. That deadline is fast approaching.
Mr. Trump has not said what steps he is prepared to take to make good on his campaign vow to hasten the defeat of the Islamic State. But he has a high regard for American generals and for Mr. Mattis, and he is likely to be receptive to their recommendations.
General Votel’s trip to the region and a visit Mr. Mattis recently made to Iraq are intended to help the Pentagon refine the plan that is presented to the White House.
The United States has about 500 Special Operations troops in Syria. If the American military presence were to be expanded, additional personnel could come from conventional combat units, though General Votel stressed that he would not recommend deploying large combat formations.
“We want to bring the right capabilities forward,” he said. “Not all of those are necessarily resident in the Special Operations community. If we need additional artillery or things like that, I want to be able to bring those forward to augment our operations.”
Raqqa has long been an objective for the American-led campaign. In addition to serving as the Islamic State’s capital, it has been a sanctuary for militants who have plotted to carry out terrorist attacks in Europe.
But the mission to seize Raqqa has been seriously complicated by Turkey’s vociferous objections to any effort by the United States to arm the People’s Protection Units, a Kurdish militia in northern Syria known by its Kurdish initials, Y.P.G.
American military officers have said that the Y.P.G. is the most capable Syrian fighting force and the best hope for mounting an attack to capture Raqqa in the coming weeks. To conduct urban warfare, however, the group needs to be equipped with armored vehicles, heavy machine guns and other arms.
Turkey, however, has denounced the Y.P.G. as a terrorist group. The United States ambassador in Ankara, American officials say, has cautioned that proceeding with the plan to arm the Kurdish group could prompt a major Turkish backlash, which could ultimately undermine American military efforts in Syria.
After months of sharp debate within his administration, President Barack Obama concluded during his final week in office that the United States should arm the Y.P.G., former administration officials said. But Mr. Obama left the ultimate decision to the Trump administration, which had informed his national security adviser that it wanted to conduct its own review of military strategy.
Many observers say that if arming the Y.P.G. is ruled out, it could take a long time to cobble together an alternative force that could draw on Turkish-backed Syrian militias and other fighters. How effective that force might be is unclear. The Turkish military and the Syrian fighters it backs have had a difficult time trying to seize the northern town of Al Bab from the Islamic State even though American teams have been inserted with Turkish units to call in American airstrikes.
General Votel did not detail how the United States might proceed if the White House ruled out equipping the Y.P.G. in deference to Turkish concerns. But he asserted there were several ways to keep up the pressure against Raqqa, including making greater use of American troops.
“We might bring potentially more of our assets to bear if we need to, as opposed to relying on our partners,” he said. “That’s an option.”
“There could be other forces that we potentially bring in to do this,” General Votel added. “It could be a different approach to how we go after the city in terms of changing our tactics.”
Toward the end of his administration, Mr. Obama approved the use of three Apache attack helicopters to support the Raqqa offensive. Expanding the use of Apaches, which have yet to be deployed in Syria, could be an option as well, observers say.
What has been successful “for us in the campaign thus far, I think, has been simultaneous pressure on the Islamic State and continuing to present them with lots of dilemmas,” General Votel said.