THE HAGUE/NAIROBI - A lawyer for Kenya's deputy president-elect William Ruto, who is charged with crimes against humanity, said a key prosecution witness had withdrawn, potentially complicating another case related to election violence five years ago.

Ruto is facing a trial at the International Criminal Court (ICC) where president-elect Uhuru Kenyatta is also charged with orchestrating ethnic-fuelled violence after the 2007 vote.

Both men deny the accusations and have pledged to clear their names.

If confirmed by the prosecution, the loss of the witness in Ruto's case could further hurt the image of the ICC and its ability to hold to account those charged with masterminding the violence.

"A significant prosecution witness got in touch through a solicitor in Nairobi saying he didn't want to be a witness any further," David Hooper, lawyer for Ruto, said on Tuesday.

He said the identity of the witness, who is covered by the Hague-based court's witness protection scheme, was confidential. Newspapers in Kenya named him.

Calls by Reuters to the prosecutor's office regarding witnesses in the Ruto case were not immediately returned.

Hooper said the prosecution had not formally acknowledged the witness's position.

"He (the witness) was claiming that someone had put pressure on him to produce false testimony," Hooper said.

"I think the prosecutors want to satisfy themselves that the witness is withdrawing on a voluntary basis," Hooper said.

Kenyatta's lawyers requested on Monday that charges against their client be dropped in part related to the withdrawal of a witness connected to his trial.

Reuters