Kenya's capital, Nairobi, has been rocked by two explosions in crowded downtown areas that have killed one person and wounded more than 20 others.

Officials say someone hurled a grenade Monday evening at a Nairobi bus stop. One person died and eight others were rushed to the hospital after the attack.

Earlier Monday, an unidentified assailant threw a grenade into a downtown Nairobi bar, wounding at least 13 people.

Police are investigating both attacks, which followed warnings from the Somali militant group al-Shabab that it would launch attacks inside Kenya.

Last week, Kenya sent troops into Somalia in pursuit of the militants, who Kenyan officials blame for the cross-border kidnapping of several foreigners.

Police say the Russian-made grenade used in the bar attack was the same type used last year in an attack on a Nairobi bus headed to Kampala. Investigators in Kenya and Uganda have linked that attack to al-Shabab.

Two days ago, the U.S. embassy in Kenya warned of an imminent terror attack in the country, although it did not specify who would carry it out. The U.S. said the terrorists would likely target places where foreigners gather, such as shopping malls and nightclubs.