Armed groups killed 173 people in Kenya in 2014, the highest number in the three years since Kenya experienced violence blamed on neighbouring Somalia's al-Shabab fighters, the country's police force has said.

The 2014 death toll is higher than that of 2013 when al-Shabab killed at least 67 people in an assault on Nairobi's Westgate Mall.

Kenyan police said on Wednesday that 312 people have been killed in the country in attacks by armed groups since 2012, and 779 people were wounded during the period.

Nairobi, Mombasa, Garissa, Mandera and Wajir were identified as towns where attacks took place between 2012 and 2014.

The targets included security officials, places frequented by tourists, passenger service vehicles, churches, shopping centres and other crowded places, said acting police chief Samuel Arachi.

Kenya deployed its troops to Somalia in October 2011 to fight al-Shabab which it had blamed for carrying out cross-border attacks including the kidnapping of four Europeans in its country.

Al-Shabab vowed retribution for Kenya sending troops to Somalia and in a video released as recently as Saturday threatened more attacks on its neighbour.

Kenyan troops are now part of a multinational African Union force bolstering Somalia's weak government against an insurgency by al-Shabab which is linked to al-Qaeda.

Al-Shabab gunmen shot dead at least 60 non-Muslims in two separate incidents in northern Kenya in late November and early December.

Border wall

A Kenyan official said on Tuesday the government is looking at new options to control the threat from al-Shabab, including constructing a wall or gullies on parts of the 682km border with Somalia.

The official, who insisted on anonymity because he is not authorised to speak to the press, said there are ongoing discussions about how to create such a buffer zone.

John Githongo, a Kenyan corruption expert, said systematic corruption prevents the country from effectively tackling such attacks.

"The systemic corruption that afflicts all our institutions infects our security services, too," he said.

"This continues to be the dead weight our efforts against terrorism have to carry."