The United Nations has been accused of breaching its principles in the Syrian conflict by effectively letting the government control aid deliveries.
More than 50 humanitarian, human rights and civil society groups back a report that says the UN has given in to demands not to help rebel-held areas.
This has contributed to thousands of civilians' deaths, the report alleges.
The UN said the report "discredits the work of national and international humanitarian aid workers".
"The UN has never shied away from calling out the parties in the conflict, including the Syrian government, for hampering the humanitarian work of the UN in Syria," Stephane Dujarric, Secretary General Ban Ki-moon's spokesman, told the Associated Press.
More than 250,000 people have been killed and 11 million others have been displaced since the uprising against President Bashar al-Assad began in 2011.