Two witnesses due to testify in Kenyan President Uhuru Kenyatta's upcoming crimes against humanity trial have withdrawn because of security concerns, the International Criminal Court said on Thursday.

Testimony by a third witness was also no longer needed to prove the case, chief prosecutor Fatou Bensouda's office said in a statement sent to AFP.

"On 16 July, the prosecution notified the Trial Chamber of the withdrawal of three witnesses," Bensouda's office said, adding "there are now a total of 30 witnesses on the prosecution's list."

Kenyatta, 51, is expected to go on trial on November 12 when he will face five counts of crimes against humanity including murder, rape and forcible transfer for his role in post-poll violence that ripped through Kenya after disputed election results in late 2007.

More than 1,100 people died and up to 600,000 others were displaced in the bloodshed, shattering Kenya's image as a beacon of region stability in east Africa.

Two witnesses -- only identified by their numbers -- cited concerns over their security and told the court to take them off the witness list.

"Witness 5 has informed the prosecution that he is no longer willing to testify," Bensouda said in a court document filed before the ICC's judges on Tuesday.

"This (willingness to testify) in his view, has created insurmountable security risks to himself," she added.

"Witness 5 had some security concerns about his cooperation with the court for some time."

Bensouda added: "Witness 426 has informed the prosecution that he is no longer willing to testify at trial."

Despite talks with the prosecution "to mitigate his concerns and secure his attendance at trial... Witness 426 maintained he was not willing to testify," Bensouda said.

The Gambian prosecutor in the past has voiced her concern about witness interference in Kenya, which her office called "unprecedented."

"Witness protection remains one of the Prosecutor's highest priorities," and Bensouda's office was working "assiduously" to address this, her office added in the statement.

What began as riots quickly turned into ethnic killings and reprisal attacks, plunging Kenya into its worst wave of violence since independence from Britain in 1963.

A fellow accused, Joshua Arap Sang, and Kenya's Deputy President William Ruto are to go on trial on September 10, also facing crimes against humanity charges.