The newspaper's analysis of hundreds of UN contracts granted since the Syrian conflict began in 2011 showed many awarded "to companies run by or linked to individuals under sanctions" from the EU and US.
The Guardian found that two UN agencies had partnered up with the Syria Trust charity, an organisation started and chaired by Assad's wife Asma, spending a total of $8.5 million (7.6 million euros).
It also said the UN had given money to the state-owned fuel supplier, which is under EU sanctions, and to Syria's national blood bank, which is controlled by Assad's defence ministry.
Money also went to the Al-Bustan Association, owned and run by Assad's billionaire cousin Rami Makhlouf, who is Syria's most notorious and powerful tycoon.
The UN's Food and Agriculture Organisation had given $13.3 million to the Syrian agriculture ministry, which is on the EU sanctions list, the Guardian said.
"These contracts show how the UN operation has quietly secured deals with individuals and companies that have been designated off-limits by Europe and the US," the Guardian said.
Reinoud Leenders, an expert in war studies at King's College London, wrote in the Guardian that the "UN's alleged pragmatism has long given way to troubling proximity to the regime".
But a UN spokesman defended the contracts.
"Operating in Syria, with the conflict now entering its sixth year, forces humanitarians to make difficult choices," the spokesman told the paper.
"When faced with having to decide whether to procure goods or services from businesses that may be affiliated with the government or let civilians go without life-saving assistance, the choice is clear: our duty is to the civilians in need," he said.
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