UNITED NATIONS - U.N. Secretary-General Ban Ki-moon voiced concern on Friday that former south Sudan rebels have withdrawn from a national coalition government and urged the parties to keep the peace process alive.

The former rebels, part of the Sudan People's Liberation Movement, withdrew on Thursday to pressure their northern partners -- the coalition's dominant National Congress Party -- to reignite a stalled peace process.

Ban called on the sides to resolve their dispute in a way that preserves a 2005 peace accord between north and south Sudan.

"The Secretary-General urges the parties, in particular the National Congress Party, to urgently take the necessary steps to address the outstanding issues related to the implementation of the (peace agreement)," his spokeswoman Michele Montas said in a statement.

The Sudan People's Liberation Movement leader, Pagan Amum, said on Thursday that the National Congress Party had failed to carry out key parts of the 2005 accord.

But that party blames divisions within the SPLM for delays in implementing the peace deal, which ended Africa's longest civil war and created a coalition government in Khartoum, with the SPLM taking just over a quarter of the posts.

The United Nations believes the 2005 peace agreement is critical for peace in all of Sudan, including the war-ravaged western region of Darfur, even though that conflict is separate.

Problems with the peace accord create further difficulties before Darfur peace talks scheduled to open in Libya on Oct. 27 between the Khartoum government and rebel groups under U.N.-African Union mediation.

Continued violence in Darfur, quarrels among the rebels and delays in putting together a planned U.N.-AU peacekeeping force have already raised doubts about the outcome of the talks.

In Washington, the State Department said it was essential that all sides fully implement the north-south agreement.

"There has been a lot of success in implementing that agreement over time but there is certainly more that needs to be done. Certainly we would encourage all the parties involved to continue to work together to implement the agreement," spokesman Tom Casey said.

Casey also voiced strong concern over an increase violence in Darfur.

"We would call on all the parties, the rebels, the government, and any associated groups to stop these kinds of attacks, to halt violence and to engage with one another in political dialogue, including in Tripoli at the conference," he said.