WASHINGTON — U.S.-backed ground operations against Islamic State remnants in eastern Syria have been put on hold because Kurds who had spearheaded combat against the extremists have shifted to a separate fight with Turkish forces, U.S. officials said Monday.

The public acknowledgment of what Col. Rob Manning, a Pentagon spokesman, called an “operational pause” is the most explicit sign yet that Turkey’s intervention in the Kurdish-held enclave of Afrin is hindering the U.S. effort to finish off Islamic State, also known as ISIS, in Syria.

For weeks, Defense Secretary Jim Mattis and other U.S. officials have called Turkey’s operation a “distraction” from the anti-Islamic State campaign. Mattis also has said the U.S. understands that Turkey has an active Kurdish insurgency inside its own borders and that it views Syrian Kurdish People’s Protection Units, or YPG, to be a terrorist organization. The U.S. says the YPG is separate from the Kurdish fighters inside the U.S.-backed Syrian Democratic Forces, but Turkey disagrees.

Turkey launched its air and ground offensive in the Afrin enclave on Jan. 20, conducting airstrikes and artillery strikes on a daily basis. It is just one dimension of a complex war in Syria that includes a range of local opposition fighters, extremist elements, Syrian government troops, proxy forces and military units from Russia and the United States.

Manning said that although ground operations against Islamic State in the Euphrates River Valley have been temporarily suspended, U.S. airstrikes against ISIS holdouts in that area are continuing. He said one airstrike Sunday near the city of Abu Kamal destroyed two ISIS supply routes.

“The nature of our mission in Syria has not changed,” Manning said. He said the Syrian Democratic Forces, which are comprised of Kurdish as well as Arab fighters, remain “our major partner” in completing the war against ISIS in Syria.

Another Pentagon spokesman, Maj. Adrian Rankine-Galloway, said he could not offer an estimate of the number of Kurdish fighters who have left the Euphrates River Valley battlefield to join the fight against Turks in Afrin.

“They’re not fighting ISIS any more, and that basically meant that they’re not taking territory back from ISIS as quickly as they had been in the past,” Rankine-Galloway said.

A U.N. convoy carrying desperately needed food and medicine to besieged civilians entered the war-ravaged eastern suburbs of Damascus on Monday, but aid agencies said Syrian authorities blocked the delivery of some of the health supplies, including trauma and surgical kits and insulin.

The shipment was the first to enter eastern Ghouta since Russia instituted what it called daily “humanitarian pauses” in the fighting a week ago. It also was the first time in weeks that any aid has been allowed in amid a crippling siege and a government assault that has killed hundreds of people in the past month.

Despite the truce, at least 50 civilians were killed Monday in airstrikes and shelling, according to the Britain-based Syrian Observatory for Human Rights monitoring group. The activist-run Ghouta Media Center said 24 people were killed in Hammouriyeh and another 10 in Harasta, both towns in eastern Ghouta.

The U.N.’s humanitarian office said the 46-truck convoy of health and nutrition supplies, along with food for 27,500 people, entered Douma around midday.

But it said the Syrian government did not allow 70 percent of the health supplies to be loaded and would not allow them to be replaced by other items.

The government routinely removes lifesaving medical supplies from aid convoys, in a pattern of denying such aid to civilians living in opposition areas. U.N. officials have complained for years about such actions by the Syrian government.