There have also been mass arrests of ethnic Somalis, particularly after three Kenyan soldiers were shot dead last month in the town of Garissa, not far from Dadaab.
The BBC's Bashkas Jugsodaay in Garissa says there were three explosions in three different polling stations in Mandera, a town near the border with Somalia, as officials were preparing for polls to open.
The two were part of the African Union mission in Somalia but crossed back into Kenya to repair their truck in the town of Garissa when they were attacked, an army spokesman said.