The only border crossing for aid between Syria and Turkey is closed.

The only crossing between Syria and Turkey that is approved by the United Nations for transporting international aid into Syria is closed 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 aid for the past nine years, as Syria remains in a state of civil war.

Turkey is a member of NATO and maintains friendly diplomatic relations with much of the world, giving it access to support and direct aid. But Syria, which is under sanctions because of President Bashar al-Assad’s regime, cannot receive direct aid from many countries. Nongovernmental organizations must deliver the aid, making the crossing from Turkey a lifeline.

Officials from the United Nations’ World Food Program said on Tuesday that the 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 for its response for now, but it will need for those to be replenished.

“If Bab al-Hawa is not functioning, technically speaking, there wouldn’t seem to be any other way to get cross-border aid into northwestern Syria,” said Julien Barnes-Dacey, the director of the Middle East and North Africa program at the European Council on Foreign Relations, referring to the region affected by the earthquake.

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 recent years at the United Nations, Russia, a supporter of Mr. Assad, has tried to block aid from Turkey to opposition areas of Syria and instead have it all distributed from the capital, Damascus, which the government controls.

Speaking at the United Nations on Monday, Bassam al-Sabbagh, the Syrian representative, called for sanctions to be removed, saying they were obstructing aid, and suggesting that aid should come through the government in Damascus instead.

Ned Price, the U.S. State Department spokesman, said in a briefing on Monday that the United States was determined to do what we can to address the humanitarian needs of the Syrian people. “We’ve done that over the course of the 12-year civil war to the tune of billions of dollars — we do that through a different process,” he said, referring to the nongovernmental groups that gain access to Syria through Turkey.

The border appeared to be closed for logistical reasons, including that workers on both sides had been affected by the quake, said Sharvan Ibesh, the executive director of the Bahar Organization, an aid group based in the Turkish city of Gaziantep, near the epicenter of the earthquake. “There is nothing clear with regards to the crossings,” he said.

Aid organizations face other logistical issues in the aftermath of the quake.

“Antakya airport and the road from Antakya to Bab al-Hawa are trashed, as is the road from Gaziantep to Antakya,” Emma Beals, an adviser at the European Institute of Peace, wrote on Twitter, referring to an airport in Hatay Province in southern Turkey.

The destruction in Turkey is likely to affect the rescue efforts in Syria.

“What is particularly bad with the current situation is that normally when a crisis happens in Syria, the situation on the Turkish side is fine,” said Mr. Ibesh, of the Bahar Organization. “But the Turkish provinces along the Syrian border are themselves afflicted as well.”

Hwaida Saad contributed reporting.