UNITED NATIONS — Syrians are dying “silently” as United Nations aid organizations face a regime unwilling to lift bureaucratic impediments and a fractured opposition unable to ensure security for access routes, the world body’s top humanitarian assistance official said.

President Bashar al-Assad’s government hasn’t agreed on providing more visas for international aid workers, streamlining customs procedures and taking steps to improve border security, Valerie Amos, under-secretary-general for humanitarian affairs and emergency relief, told the U.N. Security Council yesterday.

At the same time, infighting among about 2,000 rival armed groups within the Syrian opposition is on the rise, challenging the U.N.’s ability to navigate the rebels’ command and control structures, she said.

Amos expressed her “deep disappointment” in the U.N. Security Council for failing to pressure the warring parties in the 2 1/2-year civil war that has killed more than 100,000 people to ensure more help reaches millions of Syrians. There is “little change to report” since the council’s statement three weeks ago urging authorities to immediately facilitate expanded aid operations, Amos said.

The Security Council unanimously agreed on a nonbinding presidential statement on Oct. 2 calling for increased aid access to build on a diplomatic breakthrough reached days earlier on eliminating Syria’s chemical weapons.

“The progress we had hoped to see on the ground as as a result of that statement has not happened,” Amos told reporters after briefing the 15-member council in New York. “What we are seeing is a deepening of the crisis, more and more people affected.”

Amos called on the council to “exert influence and take the necessary action to stop this brutality and violence.” The strongest Security Council move would be to adopt a binding resolution. A French bid last year for such action was blocked by Russia, an Assad ally.

During the closed portion of her briefing, Amos set out “very specific areas” for which she wanted more support from the Security Council, U.K. envoy to the U.N. Mark Lyall Grant told reporters afterward. He said members will discuss other practical steps to assist Amos’s efforts before considering stronger measures, such as a resolution.

The latest U.N. figures show 6.8 million of the country’s 21.4 million population in need of humanitarian aid. There are 4.25 million people displaced within Syria and 2.2 million refugees who have flooded into neighboring Turkey, Lebanon, Iraq and Jordan and North Africa.

The U.N. has been unable to provide assistance to about 2.5 million people trapped in “hard-to-reach and besieged areas,” Amos said in her statement to the council.

She said the opposition’s siege of Aleppo in Syria’s northwest, the government’s siege of Damascus suburbs and fighting in Hama and Deraa illustrate the security challenges in delivering aid.

The country may be on the brink of its first polio outbreak in 14 years, while what little remains of the civilian infrastructure crumbles under indiscriminate attacks on thousands of schools, hospitals, power plants and water facilities, Amos said.

Kidnappings of aid workers and hijackings of aid convoys are becoming more frequent, making it difficult for the UN to find drivers willing to drive convoys, she said.

Humanitarian efforts in Syria are underfunded, with 54 percent of the $1.4 billion sought for this year raised for assistance inside the country, Amos said. With winter approaching, $1.8 million is needed before it gets “too late to procure and deliver” supplies, she said.