A Kenyan lawyer on Tuesday denied bribing prosecution witnesses at the International Criminal Court (ICC) war crimes trial of Kenyan Vice President William Ruto which collapsed six years ago.

The ICC has accused Paul Gicheru of playing a key role in a scheme to thwart the trial of Ruto, who is the estranged deputy of Kenyan President Uhuru Kenyatta and a candidate in Kenya's August presidential elections.

Ruto and his co-accused, broadcaster Joshua Sang, were both charged with fomenting ethnic violence after a disputed 2007 election in which 1 200 people died.

ICC judges ruled in 2016 that the two had no case to answer, although they left the door open to possible fresh charges in future, noting that the case had been hampered by political interference and threats against witnesses.

ICC prosecutor James Steward on Tuesday said Gicheru, who was in court on Tuesday, "managed and coordinated a scheme to identify, locate and corruptly influence actual and potential prosecution witnesses" though a combination of threats and bribery.

His actions led to at least four prosecution witnesses withdrawing and recanting their testimony, prosecutors said.

Gicheru is charged with eight counts of offences against the administration of justice. If convicted he faces a maximum of five years in prison or a fine.