Still, the president said Iraqi forces had made important gains in the last several weeks. Most important, they retook Falluja, a victory that he said, “got a little bit lost in the news, but that’s a big town.”

“They’re now positioning themselves so that they can start going after Mosul,” the president said. The Islamic State fighters, he said, were “on their heels, and we’re going to stay on it.”

White House officials resisted suggestions that more American troops might be needed to uproot the Islamic State, also known as ISIS or ISIL, from Mosul.

“The president has been very clear about what our mission is and what our mission isn’t,” the press secretary, Josh Earnest, said on Monday. “This is an effort to reinforce our support for Iraqi forces that are enjoying some success in driving ISIL out of strategic, important areas in Iraq, that can put them in a position to succeed on a much bigger goal: driving ISIL out of Iraq’s second largest city.”

Some of the American troops who will be stationed at the airfield, known as Qaiyara Airfield West, specialize in infrastructure projects, like building bridges, which is a technical skill the Iraqis will need for the assault on Mosul because the Islamic State has destroyed many around the city.

The new deployment comes two years after Mr. Obama said that while the United States would help Iraq reclaim territory from the Islamic State, its efforts would “not involve American combat troops fighting on foreign soil.” Since then, he has steadily increased the number of troops and given them more authority. Three service members have been killed.

In April, Mr. Carter came to Baghdad to announce that Mr. Obama had given American military advisers the approval to work closer to the front lines of the conflict with smaller units of Iraqi forces. As part of that announcement, Mr. Obama deployed an additional 217 troops.