A Kenyan man pleaded guilty on Wednesday to a grenade attack on a Nairobi bus station and being a member of the Somali rebel group al Shabaab, which Kenyan forces are pursuing in a cross-border operation.

Elgiva Bwire Oliacha, who is also known as Mohamed Seif, admitted in a Nairobi court to charges of injuring people in the attack, one of two in the Kenyan capital two days earlier.

He also pleaded guilty to other charges of illegally possessing a cache of arms, including 13 grenades, an AK-47 rifle, a machinegun, four pistols and 717 ammunition rounds.

The two grenade attacks at a bar and the bus terminus killed one person and wounded more than 20 Monday, two days after the U.S. embassy had warned of an imminent attack by al Shabaab.

Kenya sent troops into Somalia 11 days ago to fight the Islamist rebels, blamed by Nairobi for a series of kidnappings on Kenyan soil and frequent border incursions that threaten the east African country's security.

Police arrested Oliacha Tuesday and found the cache of weapons at a flat in a low-income area of Nairobi about 15 km (10 miles) from the city centre.

The charge sheet accused Oliacha, and others not before the court, of causing grievous harm to a man at a bus station in Nairobi. The man mentioned was hurt in the grenade attack at the downtown terminus Monday evening.

There was no mention in the charge sheet of the separate grenade attack on a nearby bar earlier in the day.

Oliacha was remanded in custody until Friday when he will be sentenced.

(Reporting by Humphrey Malalo and Duncan Miriri; Editing by David Clarke and David Stamp)