Concern about the timetable has created "tensions and a climate of uncertainty," the UN's envoy to DRC, Maman Sidikou, warned the Security Council on Wednesday.
"The impact of the deterioration of the security situation on the lives of civilian populations is clearly visible," he said.
The vast country is theoretically scheduled to vote for a new head of state by the end of 2017, under a hard-won accord brokered by the Catholic church last New Year's Eve.
The deal, said its supporters, ushered in the chance of a violence-free, democratic transition in one of the world's most troubled countries.
The vote would replace President Joseph Kabila, whose refusal to step down when his second and final term ended last December 20 stoked fears of a bloodbath.
But this upbeat scenario has been clouded by Wednesday's announcement by the Independent National Electoral Commission (CENI), which said balloting would be impossible before early 2019.
The panel explained that after it draws up the electoral roll - a process still far from complete in the restive region of Kasai - it needs another 504 days to prepare for the vote.
"If we accept the use of voting machines and if we change the electoral law," the delay could be reduced, a commission spokesperson told AFP.
Wednesday's announcement prompted some activists to immediately call for resistance.
"There can be no more waiting. Congolese people, the power of the street, it's now or never," pro-democracy group LUCHA reacted on Twitter.
"Whether it rains or snows, we will be voting in December this year, otherwise Kabila will unleash a civil war in this country, it will be a disaster," Jimmy, a citizen in a Kinshasa street, said on Thursday.
In New York, DRC's foreign minister, Leonard She Okitundu, on Wednesday said the movement towards elections was "irreversible - but (it is) a movement towards good elections, which can lead to a peaceful transfer of power."
Several months ago, he noted, the CENI had already warned that for "security, logistical, financial and regulatory reasons," it would be impossible to meet the December 2017 deadline.
"The timetable will be published shortly," he promised, according to remarks carried by the UN radio Okapi.
Rights monitors have repeatedly voiced concern over killings and rape in troubled parts of DRC, notably in the Kasai region and North Kivu, and about harassment of journalists and political opponents.
Between June and August this year, the UN Organisation Stabilisation Mission in the Democratic Republic of the Congo (MONUSCO) recorded 1 329 cases of violations and abuses of human rights.
Armed groups carried out 533 attacks, while state actors were responsible for 769 violations, half of which were attributed to the military or police.
Around 8.5 million people, including 5.5 million children, while the number of internally displaced persons has reached 3.8 million and more than 600 000 have sought refuge abroad..