"The SDF stopped the anti-ISIS operations because it's impossible to carry out any operation while you are being threatened by a large army right on the northern border," the Kurdish military source said.
One of the U.S. officials, who spoke on condition of anonymity, said the suspension also impacted U.S. training of stabilization forces in Syria.
It was unclear whether the pause affected every aspect of U.S.-partnered operations against Islamic State or whether there might be exceptions. The U.S. military was not immediately available for comment.
But any suspension in such activities would represent a direct setback to the central U.S. goal of helping the Kurdish-led Syrian Democratic Forces (SDF) secure territory seized from Islamic State.
It would also show how the Kurdish-led SDF are rapidly shifting their focus to the fight against Turkey -- at the cost of preparations to prevent Islamic State's resurgence.
Turkish warplanes and artillery struck Kurdish militia positions in several towns on Wednesday after the United States this week pulled back its troops from the Turkey-Syria frontier following a decision by U.S. President Donald Trump.
Trump's critics say his move cleared the way for the Turkish operation and amounted to a betrayal of America's allies, the Kurds. U.S. officials say the president felt U.S. troops should not be caught in the middle of that fight, and Trump says he is fulfilling promises to withdraw the United States from “endless” Middle East wars.
The Kurdish fighters, considered terrorists by Turkey, have described the U.S. decision as "a stab in the back." Spearheaded by the Kurdish YPG militia, the SDF has been the backbone of the U.S.-led campaign against Islamic State.
The U.S. military had hoped to train SDF and other groups to create a stabilization force of 50,000-60,000 fighters to help prevent a resurgence of Islamic State. As of last month, the U.S. military estimated it was perhaps about halfway toward that goal.
A third official told Reuters that the SDF was still guarding prisons holding some 11,000 captured Islamic State fighters, but noted that a small number of SDF forces had relocated ahead of the expected Turkish offensive.
U.S. officials have long feared that the SDF would be unable to continue guarding the facilities in the event of a major Turkish incursion into Syria.
Still, Islamic State remnants could force the SDF to battle it as well.
Jennifer Cafarella, research director at the Institute for the Study of War think-tank in Washington, said the SDF faced the likely prospect of having to fight on two fronts: against Turkish forces and remnants of Islamic State simultaneously.
"ISIS leader (Abu Bakr al-)Baghdadi is undoubtedly prepared for this moment," Cafarella said, using an acronym for Islamic State.
At the height of its power Islamic State ruled over millions of people in territory running from northern Syria through cities and towns along the Tigris and Euphrates valleys to the outskirts of Baghdad in neighboring Iraq.
But the fall in 2017 of Mosul and Raqqa, its strongholds in Iraq and Syria respectively, stripped Baghdadi of the trappings of a caliph and turned him into a fugitive thought to be moving along the desert border between Iraq and Syria.
U.S. officials have been warning for months against losing focus on Islamic State, which they believe could again become a potent insurgency.