The delay raises question marks over whether the launch of formal peace talks on ending the deadly conflict in the eastern DRC will go ahead as planned in Doha on Friday.

Under a deal reached in Qatar last month, the two sides had agreed to thrash out a package of measures, including a prisoner exchange mechanism, ahead of Friday's planned launch.

"Both sides are still working with support from Qatar to implement the terms outlined in the Declaration of Principles," the official told AFP, referring to the July 23 deal.

"This includes ongoing negotiations to establish a mechanism... for the exchange of prisoners, which has taken longer than initially anticipated," the official added, speaking on condition of anonymity because of the sensitivity of talks.

The DR Congo and the Rwanda-backed M23 agreed to a ceasefire as part of the July deal, which the official said had held, even though both sides stand accused of violations.

"Progress is being made and once there is an agreement on (the prisoner exchange mechanism), implementation of the deal is expected to pick up pace," the official said.

While the official said the ceasefire between DR Congo forces and the M23 agreed in Doha had held, clashes have continued between militias affiliated with the DR Congo government and the Rwanda-backed rebels.

On Thursday, M23 official Bertrand Bisimwa decried "massive and repeated violations of the ceasefire" by Kinshasa, and threatened retaliation.

On Wednesday, UN human rights chief Volker Turk condemned M23 for killing at least 319 civilians in eastern DRC in July.

The official with knowledge of the negotiations said it was "unclear" whether peace talks would begin on Friday as planned..

Bisimwa said neither M23 nor its political wing the AFC had been invited to a fresh round of talks in Doha.

The July deal signed in Doha followed an earlier, separate peace agreement between the Congolese and Rwandan governments inked in Washington in June.

The M23 had insisted on seeking its own peace deal with Kinshasa, saying the DRC-Rwanda agreement left out issues that still needed to be addressed.

Previous ceasefire agreements for eastern DRC have collapsed.

Neighbouring Rwanda denies providing military backing to the M23, but UN experts say the Rwandan army played a "critical" role in an offensive which saw the rebels capture the key provincial capitals of Goma and Bukavu earlier this year.