The only border crossing for U.N. aid from Turkey to Syria is hobbled.

The only crossing between Syria and Turkey that is approved by the United Nations for transporting international aid into Syria is not functioning because of earthquake damage to roads around it, according to U.N. officials, complicating an already fraught response to the unfolding humanitarian crisis.

The crossing, known as Bab al-Hawa, has been the lone link for U.N. aid to opposition-held areas in Syria for the past nine years during the country’s civil war.

Most aid to Syria flows through Damascus, the capital, which is in government-held territory. Because President Bashar al-Assad’s government has been able to tightly control what aid goes to opposition-held areas, the cross-border aid deliveries from Turkey have been a lifeline for opposition-held areas in the north.

Officials from the United Nations’ World Food Program said on Tuesday that the Bab al-Hawa crossing remained intact after Monday’s devastating earthquake, but it was not in use because roads leading there were damaged or closed. The agency said it was using stocks already inside Syria to respond for now, but those will need to be replenished.

Without Bab al-Hawa, there is little prospect of getting cross-border aid into northwestern Syria, where the crossing is, said Julien Barnes-Dacey, the director of the Middle East and North Africa program at the European Council on Foreign Relations.

A 7.8-magnitude earthquake on Feb. 6, with its epicenter in Gaziantep, Turkey, has become one of the deadliest natural disasters of the century.

“Even getting to Bab al-Hawa seems to be a huge ordeal at the moment,” he added. “It’s not as if the roads are functioning into Syria from Turkey.”

In the immediate hours after the earthquake, humanitarian groups in northwestern Syria were able to distribute some aid, because many of them maintain warehouses stocked with supplies in preparation for potential disasters or mass displacements, previously as a result of the war.

Given the devastation in both Syria and Turkey, there was confusion among aid groups and others about the exact state of the crossing and what, if any, aid may have been delivered.

The Bab al-Hawa administration said in a statement on social media on Tuesday that the crossing “remains open for aid and humanitarian movement for whomever wants to help our afflicted people.” But even if it was technically open, it appeared to be out of operation because of the damage to surrounding areas.

“Everything in that area has been condemned,” said Monzer al-Salal, the executive director of Stabilization Support Unit, an aid group that works on public service and governance in opposition-held areas of northwestern Syria.

Humanitarian groups also use three other crossings between Turkey and northern Syria to deliver aid. But while Syrian aid groups said that those crossings were open, they said they had not heard of any United Nations or other aid reaching Syria through any of those crossings since the earthquake struck. Some aid workers had used other crossings to enter Syria and assess the damage, but not to deliver aid, according to Mr. al-Salal.

Some countries friendly with the Syrian government have been sending aid directly to Damascus. Iran sent a plane to Damascus on Monday night with 70 tons of food, tents and medicine, and more were expected to arrive on Tuesday in Aleppo and Latakia, according to Iranian state media and the Syrian Red Crescent.

The quake’s vast destruction in Turkey is also hindering the aid’s flow to Syria, given that much of it would typically come from Turkey.

“You have Syrians injured on both sides of the border,” Mr. al-Salal said, referring to the millions of Syrian refugees who live in Turkey, many of them in the south near the border. “Most of the aid workers themselves have their own crises.”

Earlier Tuesday, he said he had seen messages people were sending around in Gaziantep asking for anyone with water tanks, shovels and saws to help work through the rubble in that Turkish city. “Yesterday I was inviting the aid groups to do a meeting to coordinate together, but it didn’t happen,” he said. “Everyone is still busy just trying to lift the rubble.”

Both Mr. Assad’s regime and its close ally, Russia, have opposed efforts to send aid directly to opposition-controlled areas from Turkey. They contend that all aid should go through the government in Damascus, which the United States and others have tried to bypass.

Speaking at the United Nations on Monday, Bassam al-Sabbagh, the Syrian government representative, called for Western sanctions on Syria to be removed, saying they were obstructing aid.

Ned Price, the U.S. State Department spokesman, said on Monday that the United States, was determined to do what it could to address the humanitarian needs of the Syrian people.

Hwaida Saad contributed reporting.