Dozens of activist groups opposed to Syria's regime accused the United Nations on Wednesday of "capitulating" to Damascus on aid access to desperate civilians.
The UN humanitarian coordinator for Syria, Yaacoub El Hillo, said however that while aid access was not ideal, the UN continues to "assist Syrians based on need".
The scathing 50-page report by The Syria Campaign advocacy group was signed by 55 Syrian organisations opposed to the government, including the White Helmets organisation made up of emergency responders in rebel-held areas.
Based on testimonies from current and former UN staff and other aid workers, the report alleged that the UN in Syria was "in breach of its humanitarian principles and therefore at risk of fuelling the conflict".
"There has been a systematic failure in the UN-led response," said Roger Hearn, who headed the UN's Palestinian refugee agency in Damascus until December 2011 and contributed to the report.
One anonymous UN official interviewed for the report called the organisation's work in Syria "a profoundly flawed and one-sided operation."
By the UN's count, nearly 600,000 Syrians live in besieged areas, most surrounded by government forces.
Earlier this month, the UN said the government had granted preliminary aid access to 15 of 18 besieged areas, after one was taken off the UN's list.
Damascus requires aid agencies to go through an authorisation process to deliver aid to these areas -- a request that is frequently rejected.
Since Syria's conflict erupted in 2011, government officials have threatened to revoke the visas of UN staff in Damascus if they deliver aid to areas without regime permission, TSC's report said.
It accused the UN of "choosing to prioritise cooperation with the Syrian government at all costs," allowing the regime to unduly influence UN aid strategy.
As a result, most assistance goes to government-held territory where permission is granted, instead of opposition areas where aid is most needed, the report said.
In April "88 percent of food aid delivered from inside Syria went into government-controlled territory," it said.
Reacting to the report during a visit to Beirut on Wednesday, the UN's Hillo told journalists the organisation does "not assist Syrians based on location. We assist Syrians based on need."
Sending an aid convoy to a besieged town without proper authorisation would be a "suicide mission for humanitarian workers", he said.
The report recommends that UN agencies publicly lay out conditions for continued cooperation with Damascus and halt work with the government if they are not met.
"A UN operation that violates its humanitarian principles becomes party to the conflict and stands accused of doing harm," it said.
Hillo admitted the government had "obstructed" access to some besieged areas.
"But because of it, do we condemn the rest to starvation?" he said.
Bissan Fakih, a spokeswoman for The Syria Campaign, countered: "A UN with the backbone to stand for its principles would help get aid to hundreds of thousands of Syrian civilians under siege, many of them only a few minutes' drive from where the UN is based in Damascus."
The Syria Campaign says the UN's "acquiescence" has also downplayed the desperate humanitarian situation for civilians living under government siege, including underestimating the numbers.
According to the UN, a total of 592,000 people live in besieged areas across Syria, but the report says that the number is closer to one million.
Earlier this year, an assessment by the UN's own aid coordination body, OCHA, also found that Damascus-based humanitarian agencies were "protective of their relationship with the GoS (government of Syria)."
It was "becoming clear that the Government was hindering the establishment of a proper humanitarian operation from quite early in the crisis," the March report said.
But "UN agencies were simply not willing to jeopardise their operations in Syria by taking a tougher stance with the Government."