Four Kenyan police officers have been shot dead after 40 heavily armed men, suspected of belonging to Somali militant group al Shabaab, attacked a police post.

The east African nation, which sent its troops into Somalia in late 2011 to pursue the al-Qaeda-linked militants, has suffered a string of gun and grenade attacks claimed by al Shabaab as retaliation. Last month, al Shabaab released two Kenyan government officials it had seized in a 2012 cross-border attack, after holding them in Somalia for more than a year.