The United States-led campaign to hunt down the last pockets of Islamic State militants in Syria has lost its most effective battleground partner in what American military officials fear will stall a critical phase of the offensive and leave open the door for hundreds of foreign fighters to escape.
Thousands of Kurdish fighters and commanders who make up the backbone of the Syrian Democratic Forces in recent weeks have diverted to defend Afrin, in Syria’s northwest, where other Kurdish militia are facing sharp attacks from Turkish troops.
The Kurdish-led S.D.F. was the driving force last fall in routing the Islamic State from its self-proclaimed headquarters in Raqqa and chasing insurgents fleeing south along the Euphrates River Valley to the Iraqi border.
That fight now is largely reliant on Syrian Arab fighters who make up a majority of the S.D.F. but lack the Kurds’ military organization and logistical prowess.
In congressional testimony on Tuesday, Gen. Joseph L. Votel, head of United States Central Command, described the S.D.F. as “the most effective force on the ground in Syria against ISIS.”
“And we need them to finish this — to finish this fight,” General Votel told the House Armed Services Committee.
Without the Kurds, the Arab forces and their American military advisers have largely been forced to halt clearing operations and taken up mostly defensive positions, United States officials said. That has left American air power to pick up the slack.
The development is another major consequence of the fighting that has rapidly unfurled in recent weeks in Syria’s tumultuous northwest.
It threatens not only to slow progress against several hundred Islamic State fighters who are hiding along the Euphrates River or in nearby deserts, but also could allow battle-hardened foreign fighters to escape deeper into western Syria and eventually into Turkey or Jordan — and possibly to return home to Europe or Africa to commit mayhem there, American commanders and analysts said.
Thousands of foreign fighters have already fled unfettered to the south and west through Syrian Army lines, these officials said.
“The campaign to defeat the Islamic State is at risk,” said Jennifer Cafarella, a senior intelligence planner with the Institute for the Study of War in Washington. “The S.D.F. is unlikely to clear remaining ISIS-held areas of the Euphrates River and could even begin to take losses due to the shift in Syrian Kurds’ main effort.” ISIS is another name for the Islamic State.
The turn of events has the potential to upset a triumph that President Trump has held out as one of his administration’s signature national security accomplishments so far.
“ISIS land has been largely recaptured — almost 100 percent,” Mr. Trump said last week during a news conference with Prime Minister Malcolm Turnbull of Australia. “They are on the run.”
Yet senior Trump administration officials and American commanders have been watching, with growing trepidation, as Kurdish troops and commanders divert from the fight against the Islamic State.
Initially, they sought to play down its significance. On Feb. 11, Defense Secretary Jim Mattis called the development “a distraction” and said it was “not a significant number right now.”
Maj. Gen. Jamie Jarrard, the Special Operations commander for the American-led coalition in Iraq and Syria, said in an interview in early February that the elimination of the entire caliphate was “very close.”
“Anything that disrupts us or takes our eye off that prize is not good,” General Jarrard said.
Mr. Mattis, Secretary of State Rex W. Tillerson and Lt. Gen. H.R. McMaster, the White House national security adviser, all have met with Turkish officials in recent days to seek an end to the clashes with Kurdish militia in northwest Syria and prevent the fighting from spreading.
But by last week, Heather Nauert, the State Department’s spokeswoman, acknowledged to reporters on Feb. 22 that “we can no longer fight ISIS the way that we would fully like to be able to do.”