(Adds S&P, details)

By Guled Mohamed

NAIROBI, March 10 (Reuters) - Kenya's fragile power-sharing deal to end a bloody post-election crisis suffered a setback on Monday as a row broke out over the role of prime minister in the proposed coalition government.

President Mwai Kibaki and his rival Raila Odinga signed the pact last month to end political turmoil which left at least 1,000 people dead and cost Kenya its reputation as one of Africa's most stable democracies.

The landmark deal will bring Odinga into the government by creating a prime minister's post for him.

In a reminder of the tense ties between the two parties, Odinga's Orange Democratic Movement (ODM) rejected on Monday a statement outlining the structure of the coalition by the head of the civil service, Francis Muthaura.

"The statement is causing alarm. It is mischievous. It could pose a threat to the accord," ODM spokesman Salim Lone said.

Parliament is due to meet on Tuesday to debate legislation to pass the deal into law and amend Kenya's constitution to make the agreement legal.

Critics said Muthaura's statement appeared to downgrade the prime minister's role by saying the vice-president would remain the principal assistant of the president.

It also said the prime minister who will hold a ministerial portfolio would coordinate and supervise government functions under the authority of the president.

"The interpretation of the accord should come out of a joint meeting between the two parties that signed it," Lone said.

"This is the view of the head of the civil service. We do not take this as the position of the president," he added.

Crucial discussions about the prime minister's precise powers and the distribution of cabinet positions were still ongoing, Lone said.

Although the power-sharing deal has been welcomed by Kenyans, many are sceptical that Kibaki and Odinga -- allies who fell out with each other -- can work together.

Citing the accord, ratings agency Standard & Poor's (S&P) raised its long-term outlook on Kenya to stable from negative on Monday.

But it also warned that polarisation between the two sides could persist, "negatively affecting governance".

S&P also affirmed the east African country's 'B' long-term and short-term sovereign credit ratings. (Additional reporting and writing by Katie Nguyen; Editing by Dominic Evans) (For full Reuters Africa coverage and to have your say on the top issues, visit: http://africa.reuters.com/ )

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