A report released by rights group The Syria Campaign accuses the United Nations of delivering 99 percent of its aid to government-controlled territories, and calls on the UN to "restore its impartiality".
The report, based on interviews with aid workers, including current and former UN staff, finds the UN in Syria in breach of its humanitarian principles and at risk of fuelling the conflict.
Entitled Taking Sides: The United Nations' loss of impartiality, independence and neutrality in Syria, the report documents how from the beginning of the conflict in Syria, the UN provided the Syrian government "with an effective veto over aid deliveries to areas outside of government control, enabling its use of siege as a weapon of war".
The organisation's fear of having visas revoked or being asked to leave Damascus has led to far-reaching and unnecessary capitulations to the Syrian government by the UN, the reports states, describing "a culture of compliance" among agencies in Damascus.
The UN's failure to set red lines for its operation has weakened its negotiating power with the Syrian government and allowed humanitarian aid to become party to the conflict in Syria, according to The Syria Campaign.
The report stated that "the UN has allowed the Syrian government to direct aid from Damascus almost exclusively into its territories.
"In April 2016, 88 percent of food aid delivered from inside Syria went into government-controlled territory. Twelve percent went into territories outside the government’s control."
The reported added that "in August 2015, the government directed over 99 percent of UN aid from inside the country to its territories".
UN spokesperson Stephane Dujarric, told Al Jazeera that "to describe the UN humanitarian work in Syria as not being impartial or not being fair, discredits the amazing work of our colleagues, mostly Syrians, are doing every day to try to deliver aid to the Syrian people".
"There has been a systematic failure in the UN-led response. Rather than basing its response on need, it has developed into a billion-dollar response programme that is largely controlled by the regime and its proxies," Roger Hearn, former head of UN agency UNRWA in Damascus, said.
Assaad al-Achi, director of Baytna Syria (Syria is our home), accused the UN of losing its credibility in the conflict in Syria and called on UN Secretary-General Ban Ki-moon to "restore the UN's credibility by setting conditions on its relationship with the Syrian government".
Backers of the report are calling on Ban to "draw a line" and set conditions for its relationship with the Syrian government aimed at protecting its humanitarian principles.
If those conditions are not met, then the UN must withdraw cooperation with the Syrian government, the report states.
The report was signed by groups such as the Syria Civil Defence, known as the "White Helmets", Basmeh and Zeitooneh, the Syrian Network for Human Rights, the Violations Documentation Center and dozens of Syrian humanitarian organisations, local councils and civil society groups.