The strongly-worded statement from the world body's human rights watchdog came a day after Iraq's justice ministry said that 42 people convicted of "terrorism" had been put to death over the past week.
Iraq also executed 23 people during two days in September, 20 of them convicted on "terrorism" charges.
"The justice system in Iraq is too seriously flawed to warrant even a limited application of the death penalty, let alone dozens of executions at a time," said Rupert Colville, spokesman for the UN's high commissioner for human rights.
"Large-scale executions of the sort that have been carried out on a number of occasions over the past two years in Iraq are not only obscene and inhuman, they are most probably in contravention of international law," he told reporters.
Pressed to explain why Iraq's application of the death penalty could be illegal, Colville said he could not comment on each individual case, but underlined concerns about the use of torture to extract confessions, as well as flawed court hearings.
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"Some are illegal, maybe most," he added.
The number of people being executed in Iraq has risen from 18 in 2010 to 67 in 2011, 123 in 2012 and 140 so far this year, Colville said.
The growing deployment of capital punishment comes as violence in Iraq has reached a level unseen since 2008, when the country was just emerging from a brutal sectarian conflict.
According to UN figures cited by Colville, at least 5,740 civilians were killed in Iraq from January to September this year.
That is more than double the number in 2011 and already much higher than the 3,238 civilians killed in the whole of 2012, which was itself the highest number in a single year since 2008, he said.
"In our view, the claim that using the death penalty can help deter terrorism is clearly exposed as a fallacy, given the soaring casualty rate in Iraq, which has occurred over roughly the same period as the dramatic and shocking increase in the use of the death penalty," said Colville.