A Kenyan man who pleaded guilty to a grenade attack earlier this week in Nairobi has been given life in prison, while Somali insurgent group al-Shabab continues to threaten more violence across Kenya in retaliation for its offensive into Somalia.

Elgiva Bwire Oliacha confessed to being a member of al-Shabab, and admitted his role in one of two grenade attacks that took place in the Kenyan capital earlier this week, which killed one person and injured more than 20 others.

Meanwhile, an al-Shabab militant leader in Somalia is calling on his followers to carry out huge explosions inside Kenya. Sheikh Muktar Robow says tossing grenades is not enough, and urges the militants to strike what he calls “big, painful blows.”

Kenyan military forces in Somalia encountered their first direct clash with al-Shabab on Thursday. The army says it killed nine Islamist fighters after al-Shabab ambushed a military convoy near the southern Somali town of Qoqani.

Kenya sent forces into Somalia earlier this month in pursuit of al-Shabab, which Kenyan officials blame for the cross-border kidnapping of several foreigners.

A Kenyan government spokesman said Thursday Kenya's goal is to destroy al-Shabab in the shortest time possible. He said the militants present a clear and present danger to the region.

At least four Kenyan government workers were killed Thursday when their vehicle was hit by a rocket-propelled grenade in northeastern Kenya, near the Somali border.