A senior Kenyan election official was found murdered on Monday, three days after he went missing, poll officials said, as opposition leaders warned the killing could plunge next week's national vote into turmoil.

Chris Msando, the election board's head of information, communication and technology, was tortured before he died, the board told journalists, without giving further details.

Kenyans - who saw their 2007 election descend into ethnic violence - will chose their next president, lawmakers and local representatives on Tuesday next week.

"There's no doubt that he was tortured and murdered," Wafula Chebukati, the chair of the Independent Electoral and Boundaries Commission, told journalists outside the City Mortuary.

"The only issue is who killed him and why ... I demand from the government that they provide security for all members of the IEBC for them to give Kenya free and fair elections."

Kenya's main opposition group, the National Super Alliance (NASA), which has repeatedly accused the government of trying to rig elections, immediately called a news conference.

"It is telling that the key person who was perhaps holding very vital passwords has been eliminated at this delicate time," NASA's Musalia Mudavadi said.

"Chris Msando's brutal killing was an attempt to drive a dagger into the heart of the forthcoming election."

Police said Msando's body was found on Saturday and brought to a mortuary.

"We view this incident as a crime of grave proportions and a ... special homicide team has been set up to investigate the murder," Kenya's National Police Service said in a statement.

Otsieno Namwaya, Africa researcher at Human Rights Watch, called for an urgent investigation, saying: "Msando's killing comes as the electoral management body was due to audit its systems, a week away from the election day."

Kenyan social-political activist Boniface Mwangi (C) talks to a street hawker during his political campaign ahead of the August elections (Reuters)

The audit, scheduled for Monday, was cancelled, Chebukati said. Msando's predecessor was suspended after he refused to cooperate in an audit of the electronic systems, local media reported in May.

Rashid Abdi, regional analyst at International Crisis Group, said public confidence in next week's vote could be affected.

"This is someone who was involved in a critical component of the elections - the electronic infrastructure. This will definitely raise suspicious and undermine public confidence in the outcome," he said.

President Uhuru Kenyatta, seeking a second and final five-year term, is running against veteran opposition leader Raila Odinga.

Odinga says that fraud robbed him of victory in the last two elections. In 2013, electronic voting machines suffered widespread malfunctions, but Odinga took his complaints to court, which dismissed them.

In 2007, he called for street protests after tallying was abruptly stopped and a winner announced. Political protests and ethnic violence killed more than 1,200 people.

The announcement of Msando's death came days after an attacker killed a policeman outside the vice president's country home.