Fresh problems held up efforts by the
"The agreement is ready, but this morning all the groups came with new demands for details to be included in the accord, which is delaying the signing ceremony," the source, speaking on condition of anonymity, said.
Talks between the government and opposition aim to resolve the future of President Joseph Kabila, who is holding onto power although his second and final five-year term ended on December 20.
The negotiations, launched on December 8, are taking place under the aegis of the influential Catholic Church, which had initially set Christmas Day as a deadline for a deal.
Representatives from both sides met on Saturday at the headquarters of Congo National Episcopal Conference (CENCO).
On Saturday, CENCO president Marcel Utembi said the two sides "are on the verge of concluding an accord."
"The various delegates have reached agreement on the points where divergences lay," he said, adding that the pact "is set to be signed tomorrow morning".
The working document for the deal, seen by AFP, envisages a "political transition" with fresh presidential elections to be held at the end of 2017.
The vote was supposed to be organised in 2016. The government had previously said it was impossible for it to be held before April 2018.
The draft deal also guarantees that Kabila will not seek a third mandate and lays the groundwork for a "national transition council" charged with carrying out the agreement.
In return, the opposition headed by 84-year-old Etienne Tshisekedi would accept that Kabila can stay in power until he hands over to an elected successor. It had previously demanded his immediate departure from public life.
Vast and resource-rich but poor and chronically unstable, DR Congo has never witnessed a democratic transfer of power following polls since independence from Belgium in 1960.
Two decades ago, the country collapsed into the deadliest conflict in modern African history.
Its two wars in the late 1990s and early 2000s dragged in at least six African armies and left more than three million dead. Its restive east remains a battleground for rival ethnic militias.
Kabila, 45, has been in power since the 2001 assassination of his father Laurent at the height of the Second Congo War.
He was confirmed as leader in 2006 during the first free elections since independence from Belgium in 1960, and re-elected for a second term in 2011 in a vote marred by allegations of massive fraud.
He obtained a controversial court ruling in May 2016 stating that he could remain in power until a successor was chosen.
The deadline for his departure from office unleashed clashes that have left between 56 and 104 people dead, according to divergent tolls.