The world's newest nation won independence from Sudan in 2011, but was plunged into a civil war two years later that killed some 400,000 people.
A peace deal in 2018 brought together President Salva Kiir and his bitter rival, Vice President Riek Machar, but their feuding has repeatedly delayed efforts to write a constitution and hold elections.
Last week, Kiir's office announced that voting planned for December would be pushed back by another two years.
International supporters have grown increasingly exasperated at the delays.
"Two years ago, we were in a similar situation as we are today and gave our support specifically under the condition that there would be no more extensions," said UN special representative Nicholas Haysom in a statement.
"Today, it is sadly evident that the country is not ready for elections that we could confidently expect to generate a credible and peaceful outcome."
Haysom said South Sudan's leaders had only achieved one of 10 priority areas laid out by international bodies last year to prepare for the vote.
"Urgency rather than a pause is required," he said.
"The South Sudanese political class must take responsibility for the failures of the past two years," Haysom added.
Embassies of eight Western nations, including the US, Britain and the European Union, also voiced disappointment at the delay and called for the vote to be held "as soon as possible".
"A 24 month extension would mean 24 months more expenditure on the institutions that implement the peace process," they said in a joint statement.
South Sudan has battled flooding, hunger and violence, while its leaders have appeared reluctant to risk going to the polls and are accused of massive corruption.
Earlier this month the UN's humanitarian agency warned more than 700,000 people had been impacted by flooding, with aid failing to meet many in need.
South Sudan boasts plentiful oil resources, but the vital source of revenue was decimated in February when an export pipeline was damaged in war-torn Sudan.
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