"The delay in releasing the results of the elections can lead to suspicions and compromise peace and stability of the country", South African President Cyril Ramaphosa and Zambian counterpart Edgar Lungu said in a joint statement.
Any widespread perception the election has been stolen could set off a destabilizing cycle of unrest, particularly in the volatile eastern borderlands where Fayulu enjoyed some of his strongest support.
On Tuesday, Fayulu and six other presidential candidates issued a statement saying that "the electoral results can not be negotiated and under no circumstances will we or the Congolese people accept such results".
Congo's powerful Catholic church challenged the surprise win of opposition leader Felix Tshisekedi in the presidential election on Thursday, saying official results do not match the outcome compiled by its 40,000 observers at all polling stations across the troubled country.
Congo and the world now face the uncomfortable choice of accepting what could be the country's first peaceful, democratic transfer of power or raising a protest that could lead to violence. But one Congolese election observer group, Symocel, on Tuesday reported "major irregularities" including the disappearance of envelopes containing results from almost 120 polling stations in Kinshasa, an opposition stronghold.
The current president, Joseph Kabila, is stepping down after 18 years in office.
The election outcome was initially expected to be announced on Sunday - and results are now expected later on Wednesday.
As the lengthy results were read out on nationwide television, police were deployed at strategic spots in the capital Kinshasa where, for the second evening running, many residents went home and locked their doors early.
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Felix Tshisekedi of Congo's Union for Democracy and Social Progress opposition party won the long-delayed presidential election.
The Independent National Electoral Commission has declared Tshisekedi (55) as victor of the 30 December poll with 38,57 percent of the vote, ahead of fellow opposition leader, Martin Fayulu, who ammassed 34,8 percent. Long in the shadow of his father, the late opposition leader Etienne, he startled Congo past year by breaking away from Fayulu - the opposition's unity candidate - to stand on his own. This is an ugly swindle of [DRC's Electoral Commission President] Mr. Nangaa and his political camp. He has two days after the announcement of official results to file official disputes.
"How long are we going to negotiate results?" said Fayulu. The diplomats spoke on condition of anonymity because they were not authorized to speak to the press. "But if the history of the Kabila regime and its tight control on the state and its security apparatus are any indication, the ... new president-elect is likely to end up on the losing end of this bargain".
Fayulu alleges that Kabila engineered a backroom deal with the largely untested Tshisekedi to protect his power base in a country with staggering mineral wealth. Backed out of an opposition deal to have a unity candidate and ran on his own ticket with the backing of politician Vital Kamerhe.
The candidate Kabila hand-picked to succeed him, loyalist former interior minister Emmanuel Ramazani Shadary, came third in the poll.
Mr Kabila took over from his assassinated father Laurent in 2001.
Elected in 2006, Joseph Kabila secured another term in controversial elections in 2011.
The preliminary results of the December 30 vote had been expected on Sunday, but the commission indefinitely delayed the announcement, to the growing suspicion of many Congolese.