The International Criminal Court on Monday dropped all charges against Kenyan civil servant Francis Muthaura, who was accused alongside president-elect Uhuru Kenyatta of crimes against humanity during 2007-2008 post-election violence.

"I have decided, as of the state of evidence available now, that we have no other choice but to withdraw the charges against Mr Muthaura," ICC chief prosecutor Fatou Bensouda told the Hague-based court.

Four prominent Kenyans, including first-round presidential poll winner Kenyatta, faced charges at the ICC over deadly post-poll violence five years ago during which prosecutors say more than 1,100 people died.

But the ICC's Bensouda said that in Muthaura's case some witnesses were refusing or unable to provide vital evidence.

"The witnesses that had conveyed evidence have been killed or died, others refuse to speak to the prosecutor," Bensouda said.

"We do not feel there are reasonable chances of a condemnation at the moment of the trial," she said.

Kenyatta and Muthaura both faced five counts of crimes against humanity for their alleged involvement in fomenting the wave of violence that swept through Kenya in December 2007 and early 2008.

The Kenyan opposition disputed the outcome of a presidential vote, unleashing the worst unrest in the east African country since independence in 1963.

More than 663,000 people were displaced in Kenya's Rift Valley after fights between rival supporters, prosecutors said, when politically motivated riots soon turned into ethnic killings, which in turn sparked further reprisals.

The clashes destroyed Kenya's image as a beacon of stability in the region, hurt its tourism industry and exposed long-simmering ethnic rifts among its population.