A UN humanitarian official warned on Wednesday that funding challenges might restrict aid to Syrians, during a visit to the country set to mark 13 years of civil war.

"We're not getting particularly good signals about funding possibilities this year," said David Carden, the United Nations deputy regional humanitarian coordinator for the Syria crisis.

"The climate for financial support, the funding environment, has become more difficult" as the war nears its 13th anniversary later this month, he told AFP on a visit to Syria's rebel-held northwest.

Insufficient funding would affect aid delivery as well as "the ability of the United Nations and its partners to provide services to the most affected people", Carden said.

Last year, the humanitarian response programme for Syria received less than 40 percent of the $5.4 billion needed to cover its operations, he said, despite a devastating quake that ripped through northern Syria and neighbouring Turkey.

The funding in 2023 was "the lowest" in terms of percentage since the launch of the UN response plans, Carden said.

More than 15 million people require humanitarian aid across Syria, according to the UN.

In Syria's rebel-held north and northwest more than four million people are in need, almost half of them children, the UN says.

One million children are out of school in the region, including 200,000 minors who dropped out following the 2023 quake, Carden told AFP.

The 7.8-magnitude tremor on February 6, 2023 killed nearly 60,000 people in Turkey and Syria, including around 1,400 people in government-held areas of Syria, according to Damascus, and more than 4,500 in rebel-held regions.

Syria's war has claimed the lives of more than half a million people and displaced millions since it broke out in March 2011 with Damascus's brutal repression of anti-government protests.