Further delays and attempts to hold onto power will only serve to isolate the DRC's leaders and government," she said.
The UN Security Council has been pressing Congolese authorities to announce a timetable for the elections, which could pave the way for the first peaceful transition of power in the vast resource-rich country.
But Tshisekedi lacks the crowd-rousing power of his father, Etienne Tshisekedi, the opposition's historical leader, who died in Brussels in February.
As political violence escalated, the Catholic church brokered a deal on New Year's Eve under which there would be a government transition, culminating in fresh elections in 2017.His ally Katumbi, a wealthy businessman and former governor of Katanga province who is president of the major football club TP Mazembe, claims to have unrivalled popularity across the country.But tactically, Katanga is on weak ground - he is in de-facto exile in Brussels, after being condemned in absentia in a real-estate affair.The DRC crisis is being followed intensely from abroad.The country is a vast reserve of resources, including diamonds, cobalt, uranium and copper, and straddles central Africa, bordering nine countries.Elections were scheduled to be held by the end of 2017, under a political deal with the opposition aimed at avoiding fresh political bloodshed after Kabila refused to step down when his second mandate ended last December."While regretting that these crucial polls have once again been postponed, we continue to call on political leaders, on all sides, to place the interests of their country and people above all else and ensure the holding of credible, free, and fair elections," said UN spokesman Farhan Haq.The electoral commission said provisional results of the presidential election would not be published until a week after the voting, on December 30, 2018, with definitive results not issued until January 9, 2019.The east of the country remains in the grip of unrest, but armed Congolese groups seem unlikely these days to threaten the regime - as they did in 1997, when Kabila's father, Laurent Kabila, overthrew president Mobutu Sese Seko with help from Rwanda.In domestic politics, the country's two biggest opposition leaders - Moise Katumbi and Felix Tshisekedi - have forged a strategic alliance.He has said he will return to DRC in December, but not spelt out how he can avoid prison.Another plus for Kabila is that he remains supported by key parts of the security apparatus, including the elite presidential guard and intelligence agency, and by his entourage.