The Kenyan suspect arrested after two grenade blasts exploded in Nairobi says he is a member of the Somali militant group al-Shabab.

Elgiva Bwire Oliacha, 28, pleaded guilty in court on Wednesday to nine charges, including being responsible for Monday's twin grenade blasts.

The blasts came about a week after hundreds of Kenyan forces moved into neighboring Somalia to attack al-Shabab militants.

Al-Shabab, Somalia's most dangerous militant group, threatened to carry out terror attacks in Kenya in retaliation.

Police arrested the suspect on Tuesday and said that he was from a Kenyan tribe and was not ethnic Somali.