The crackdown drove more than 730,000 Rohingya Muslims to flee to Bangladesh. UN investigators have said the operation was executed with "genocidal intent" and included mass killings, gang rapes and widespread arson.
Myanmar denies widespread wrongdoing and says the military campaign across hundreds of villages in northern Rakhine was in response to attacks by Rohingya insurgents.
"Without question serious errors were committed and opportunities were lost in the UN system following a fragmented strategy rather than a common plan of action," wrote former Guatemalan foreign minister and UN ambassador Gert Rosenthal in a 34-page internal review, seen by Reuters.
"The overall responsibility was of a collective character; in other words, it truly can be characterised as a systemic failure of the United Nations," wrote Rosenthal, who was appointed by Secretary-General Antonio Guterres earlier this year to look at UN involvement in Myanmar from 2010 to 2018.
He said senior UN officials in New York could not agree on whether to take a more robust public approach with Myanmar or pursue quiet diplomacy and that conflicting reports on the situation were also sent to UN headquarters from the field.
The United Nations struggled to balance supporting the Myanmar government with development and humanitarian assistance, while also calling out the authorities over accusations of human rights violations, Rosenthal concluded.
"The United Nations system ... has been relatively impotent to effectively work with the authorities of Myanmar to reverse the negative trends in the area of human rights and consolidate the positive trends in other areas," he said.
"The United Nations' collective membership, represented by the Security Council, bears part of that responsibility, by not providing enough support to the secretariat when such backing was and continues to be essential," Rosenthal wrote.
The 15-member Security Council, which visited Myanmar's Rakhine state last year, has been deadlocked with Myanmar allies China and Russia pitted against western states over how to deal with the situation.
UN spokesman Stephane Dujarric said Rosenthal's report was due to be sent to all 193 UN members states on Monday and would then be posted publicly online.
"Its conclusions and observations have been fully accepted by the Secretary-General, and he will work very closely with the senior leadership to make sure they're implemented," he said.