The brother of a man arrested for allegedly being involved in an extremist plot to attack President Uhuru Kenyatta's rural home says he is mentally ill.

Ahmed Mire says his 46-year-old brother Said Mire Siyad has been receiving psychiatric treatment for chronic schizophrenia at a government hospital since 2004.

The 26-year-old Mire says Siyad was staying with him and left home on Tuesday and didn't return.

Police claim Siyad was carrying out surveillance of Kenyatta's home in central Kenya in preparation of an attack.

Kenya security forces have increased the frequency of arrests of people suspected of involvement in extremism since four gunmen from the Somali extremist group al-Shabab killed 148 people earlier this month at a college in Garissa.