Syria's civil defense units slammed the United Nations for failing to properly respond to the humanitarian crisis in the opposition-held areas of the country, destroyed by two major earthquakes.

Raed al-Saleh, who leads the White Helmets, said the area had not received any aid from the U.N. since Monday's earthquake aimed at disaster response, saying that the six trucks that crossed the border into Syria on Thursday had been a regular shipment that had been delayed.

"The trucks that entered yesterday are a convoy that was scheduled to enter on Monday but was late due to the earthquake," he told reporters via video link from Idlib. "Until now no aid has arrived to northwest Syria from the U.N. as a response to the earthquake."

Al-Saleh called the United Nations' response "catastrophic" and said the body should "apologize to the Syrian people for the lack of help it provided."

The U.N. Office for the Coordination of Humanitarian Affairs did not immediately respond to a request for comment on al-Saleh's allegations.

The International Organization for Migration (IOM), a U.N. agency, said 14 trucks carrying humanitarian aid had crossed into Syria after departing from the southern Turkish city of Gaziantep earlier on Friday. The trucks, bound for opposition-held Idlib, carried items including electric heaters, tents and blankets, according to the IOM.

When asked whether the 14 trucks that crossed into Syria were part of a regular aid shipment and not linked to the earthquake response, IOM spokesperson Paul Dillon said the "pre-positioning" of aid was not the issue.

"The issue is that critically needed humanitarian aid that is suitable for people who have been displaced, including tents, blankets and other materials, are being delivered to northwest Syria at this time," he said.

The White Helmets, known officially as Syria Civil Defence, have been credited with saving thousands of people in opposition-held areas hit by bombing by regime and Russian forces in Syria's 12-year-long civil war.

White Helmets members say they are neutral. Syrian regime leader Bashar Assad and his backers, including Russia, describe them as tools of Western propaganda and insurgents.

World forgot about Syria: WHO official

Meanwhile, a senior World Health Organization (WHO) official bemoaned Syria's "forgotten crisis" on Friday, as aid began trickling into rebel-held areas, days after a devastating earthquake.

As the WHO prepared to fly medical supplies to Syria from Dubai, Mike Ryan, executive director of the WHO's Health Emergencies Programme, said a huge backlog of aid was waiting to reach Syria's opposition-held northwest.

The single aid corridor bypassing regime-controlled areas and servicing the war-shattered region, which is home to 4 million people, was temporarily cut off by Monday's massive quake.

The first convoy of emergency supplies finally passed through the Bab al-Hawa border crossing from Türkiye on Thursday. More than 22,000 people died in the disaster in both countries.

"The world's forgotten about Syria," Ryan told reporters in Dubai, during preparations for the aid flight.

"Frankly, the earthquakes brought attention back. But those millions of people in Syria have been struggling now for years. That's become a forgotten crisis."

U.N. chief Antonio Guterres urged the Security Council on Thursday to authorize the opening of additional crossings on the Türkiye-Syria border for the delivery of U.N. aid to opposition areas.

This week, WHO chief Tedros Adhanom Ghebreyesus warned of a "race against time" to save lives in the quake zone as aftershocks continue and freezing winter conditions bite.

Even before the earthquake, a decade of civil war and Syrian-Russian aerial bombardment had destroyed hospitals and prompted electricity, fuel and water shortages in the opposition-held northwest.

"There's a huge backlog of supplies ready to go," Ryan said. "Many agencies, including our own had pre-stocked because we're into the winter, they're already very exposed.

"There's a huge problem with millions of people."

The chances of finding survivors had dimmed before the first aid arrived, after the end of the three-day period that experts consider critical to saving lives.

Syria is now facing a "secondary disaster" of lives lost due to a lack of medical supplies, Ryan warned.

"We have to recognize the scale of this disaster is so large, it's overwhelming everyone's capacity," he said.

"If they don't have equipment, they can't do their job – it's like asking a fireman to rush to a fire without a fire hose."

Officials and medics said 19,875 people had died in Türkiye and 3,377 in Syria in Monday's quake, bringing the confirmed total to 22,368. Experts fear the number will continue to rise.