Defense officials said that only 100 to 120 Syrian fighters were in training right now.

“So we’re counting on our fingers and toes at this point when we had envisioned 5,400 by the end of the year,” Ms. McCaskill said.

The White House acknowledged that the program had not succeeded. “The administration knew on the front end that this would be a quite difficult task, and it’s proved to be even more difficult than we thought,” Josh Earnest, the White House press secretary, said on Wednesday.

Mr. Earnest tried to turn the tables on critics of the administration by noting that they had long argued in favor of American training of Syrian rebels.

“Many of our critics had proposed this specific option as the cure-all for all of the policy challenges we’re facing in Syria right now,” he said.

General Austin also told the committee that he could not comment on an internal Pentagon investigation into whether senior military officers manipulated the conclusions of reports on the war against the Islamic State, because the investigation was in progress. He said that “once the investigation is completed, based on the findings, you can be assured that I will take appropriate actions.”

Separately on Wednesday, France’s defense minister said that French forces would join the coalition of Western and Middle Eastern countries carrying out airstrikes on the Islamic State in Syria, with the first strikes likely to come in the next couple of weeks.

The airstrikes would represent an expansion of France’s military activity in the region, where it has already been involved in bombing Islamic State targets in Iraq and began reconnaissance flights in Syria last week.

The Islamic State has control of much of the territory that extends from northern Syria across the Iraqi border to Mosul and south to within about 30 miles of Baghdad.