Dozens of Syrian aid groups announced they were suspending work with the United Nations, saying some UN agencies and partners have been manipulated by the Syrian regime.
A total 73 groups signed the letter to UN Office for the Coordination of Humanitarian Affairs that also demanded an investigation of UN agencies' work in Syria.
The signatories included the Syrian American Medical Society and the Syrian Civil Defense, known as the White Helmets, who are active in opposition-held areas.
A spokesman for the UN, Stephane Dujarric, rejected those assertions and earlier press reports that some UN aid contracts had gone to close allies of the Syrian regime.
Meanwhile, in the ravaged city of Aleppo, government forces and their allies captured new ground on the city's edge, tightening the siege on rebel-held parts of the city.
More than 290,000 people have been killed in the war, now in its sixth year, and international efforts to end the conflict have faltered.
Based on reporting by AFP and AP