One person was killed and others wounded in a grenade attack at a Nairobi bus stop [Reuters]

A Kenyan suspect arrested after two grenade blasts this week in capital Nairobi has admitted in court that he was involved in one of the attacks, and said he is a member of the Somalia-based armed group al-Shabab.

Elgiva Bwire Oliacha, 28, also known as Mohammed Seif, pleaded guilty to nine charges on Wednesday, including over Monday's explosion at a busy bus stop which left one person dead and several more injured.

The grenade blasts came after the al-Qaeda-linked group warned it would attack Kenya in retaliation for its military incursion into southern Somalia.

Some though, questioned whether al-Shabab was behind the attacks given that it had already demonstrated it was capable of killing dozens of people in attacks on foreign soil.

An officer with the anti-terror police unit who brought Oliacha to court on Wednesday said authorities would bring more charges against him in court on Friday, including charges related to the grenade attack on a pub early on Monday morning that wounded a dozen people.

The officer, who wanted to remain anonymous, said that Oliacha was expected to plead guilty to all of those charges as well.

During his arrest he was found with six guns, 13 grenades and hundreds of bullets in his house in a slum called Kayole in eastern Nairobi, police said. Oliacha admitted to possession of the weapons in court.

Chief magistrate Gilbert Mutembei allowed police to continue holding Oliacha for two more days to complete their investigations.

Monday's blasts came days after hundreds of Kenyan forces moved into Somalia to attack al-Shabab fighters who the government blames for a string of kidnappings, including aid workers and European tourists, in recent weeks, from northern Kenya. Al-Shabab has denied involvement in the kidnappings.