“Thus far, diplomatic efforts designed to end years of suffering have failed,” the statement said.

United Nations officials said that Valerie Amos, the under secretary general for emergency relief and one of the signatories of the statement, would brief members of the Security Council at the end of April on the implementation of the Feb. 22 resolution demanding that humanitarian agencies have unfettered access to civilians. The resolution did not commit to imposing sanctions for noncompliance, but the Council warned that it would take “further steps” against parties that disobeyed.

“All too often, humanitarian access to those in need is being denied by all sides,” the agency chiefs said. The statement was signed by Ms. Amos; António Guterres, the United Nations high commissioner for refugees; Anthony Lake, the executive director of Unicef; Ertharin Cousin, the executive director of the World Food Program; and Margaret Chan, director general of the World Health Organization.

“At least one million people are now in need of urgent humanitarian assistance in Aleppo alone,” the agency heads stated, noting that the vital road link between that city and Damascus had often been cut and that “other key roads are blocked by different armed forces and groups.”

Since the Security Council passed its resolution, some aid convoys have reached parts of northern Syria from Turkey and the northeastern province of Hasakah from Iraq. Agencies also managed this month to deliver aid to Aleppo for the first time since June. The aid organization chiefs said, however, that the amount of assistance that was getting through was “not nearly” enough.

Fighting around the Damascus suburb of Yarmouk has blocked deliveries of food and other aid to around 18,000 residents for more than two weeks, according to the United Nations, which has received widespread reports of “women dying in childbirth for lack of medical care and infants, the elderly, women, the sick and the dying reduced to eating animal feed.”