Kenya's government has agreed in principle to the creation of a prime minister's post sought by the opposition, but the terms and duties are still being discussed, a government negotiator said on Thursday.

"That is more or less agreed on. What we are discussing now is the post's functions, responsibilities, nature of appointment and so on," Mutula Kilonzo, a negotiator at talks led by former U.N. chief Kofi Annan, told Reuters. "This will be an interim measure."

The opposition has demanded the post and a 50-50 split in cabinet positions.

It has threatened new street protests if there is no political solution within a week -raising fears of more bloodshed after violence over the disputed December 27 election that has killed more than 1,000 people.

A senior opposition official, speaking on condition of anonymity, said there was discussion of two roles for the prime minister.

"One of them is some sort of chief minister ... or there is a head of government who is answerable to parliament and who is appointed and his conduct is regulated by the national assembly. You have a real separation of powers," the official said.

Asked if the position would have executive powers, Kilonzo said: "We will not give anybody a hollow shell."

He said the interim post would come with a sunset clause that causes the position to expire either when parliament is dissolved or when a new constitution is enacted.