The ICC said in a statement Tuesday that it was deeply concerned and stands ready to assist Kenyan investigations. It said it had offered the witness security measures.
The body of Meshack Yebei, who disappeared on December 28, was found in a river in Nandi county, some 300 km west of Nairobi, on January 4. His skull had been smashed and his teeth broken.
Kenya's Director of Public Prosecutions Keriako Tobiko has ordered police to carry out a "speedy and thorough investigation" into Yebei's murder. Ruto's lawyer Karim Khan has written to the head of Kenya's Criminal Investigation Department saying Yebei had been a "critical witness" for the defence, who had been referred to the ICC's Victims and Witness Unit for protection.
"It's our strongly held view that an attack on any witness constitutes an interference with the proper administration of justice and indeed an attack against William Ruto," said Khan, who also called for DNA tests to confirm the identity of the body.
"The Court is profoundly concerned by this grave reported incident,” said ICC Registrar Herman von Hebel. “It stands ready to provide the local authorities with any assistance, if required, in their investigations. Ensuring the safety and security of witnesses is a cornerstone of fair trials."
The ICC Registry had offered Mr Yebei security measures, including a safe residency in a new location, according to the Court. However, he returned to Eldoret where the incident reportedly took place.
“Mr Yebei was not on the Prosecution witness list nor was he in contact with Prosecution staff at the time of his abduction,” the ICC stressed.
The trial of Kenyan Deputy President William Ruto and journalist Joshua Arap Sang opened before the ICC in September 2013
The two men are accused of crimes against humanity for their alleged role in deadly violence that followed elections at the end of 2007.