KINSHASA (Reuters) - A deal struck last month requiring Congo President Joseph Kabila to step down after elections this year risks unraveling if politicians do not quickly reach compromises on implementing the accord, Catholic bishops mediating the talks said on Monday.

The Dec. 31 deal was greeted as a critical step toward averting a slide into anarchy and possibly civil war in Democratic Republic of Congo over Kabila's decision to remain in power when his mandate expired last month.

The accord, signed by representatives of Kabila's ruling coalition and the main opposition bloc, bars Kabila from trying to change the constitution to stand for a third term in an election to be held by the end of this year.

But talks this month on implementing components of the deal have stalled, Congo's Catholic Bishops Conference (CENCO) said in a statement.

"The CENCO launches an appeal to the negotiators to ... not lose sight of the main objective of these negotiations, which is organizing the elections in less than a year," it said.

"The CENCO is not prepared to mediate indefinitely without results."

The main obstacles include a disagreement over the composition of a council to monitor progress toward elections and whether the main opposition bloc must allow Kabila to choose from multiple prime ministerial candidates.

Kabila has ruled the giant central African country since his father's assassination in 2001 and the extension of his mandate saw violent protests in which security forces killed at least 40 people.

Congo has never experienced a peaceful transition of power and millions have died in conflicts in the country's east since 1996, most from hunger and disease.

Also on Monday, Congo's government spokesman Lambert Mende told Reuters Human Rights Watch's (HRW) Central Africa director Ida Sawyer was expelled from the country for falsifying her name on immigration documents.

Sawyer, a vocal critic of rights abuses by authorities, returned to Congo last week after being forced to leave in August when the government refused to renew her visa.

HRW said in a statement Sawyer had a valid visa and quoted executive director Kenneth Roth as saying the expulsion "throws into question the Congolese government's commitment to reversing the climate of repression that reigns in the country."

(Reporting by Aaron Ross; Editing by Joe Bavier and Janet Lawrence)