HAMILTON, Canada

The UN relief chief described Sudan as the world's worst humanitarian crisis, warning that international failure to act risks defining this era by "indifference and impunity."

"Again and again, the international community has said that we will protect the people of Sudan," Tom Fletcher said Thursday in a statement, adding "the people of Sudan should ask us if, when and how we will start to deliver on that promise."

"This is the world’s largest humanitarian crisis," he said, noting that more than 30 million people -- over half the country's population -- are in urgent need of aid, as indiscriminate violence from Kordofan to Darfur continues "without mercy."

Citing mounting civilian deaths, the collapse of Sudan's health system and the onset of the lean season, Fletcher warned that diseases like cholera and measles are spreading rapidly.

"Our appeals are pitifully supported," he said, asking: "Where is the funding?"

Fletcher also condemned repeated attacks on hospitals, infrastructure and aid convoys, including last week's deadly assault on a UN humanitarian convoy in North Darfur.

"Talk has not translated into real protection for civilians or safe, unimpeded and sustained access for humanitarians," he said, adding: "Where is the accountability?"

“We call on all with influence to step up,” he said. “Protect civilians. Guarantee safe access for humanitarians. Fund their work. Insist on agreements to humanitarian pauses and other arrangements that can allow us to safely reach the areas and people worst hit. Work harder to secure a lasting, inclusive and just peace.”

"Let this time not be defined by indifference and impunity but by a revival in human solidarity for those in greatest need, and determination to hold to account those responsible for it," he added.