Police in Kenya say it is too early to tell if a grenade attack at a downtown Nairobi bar is linked to al-Shabab, the Somali militant group that recently threatened to strike inside Kenya.

Authorities say an unidentified assailant threw a grenade inside the bar early Monday, wounding 13 people.

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

The al-Qaida-linked group has warned it will retaliate against Kenya for sending its troops into Somalia to pursue al-Shabab fighters.

Kenyan authorities believe the militant group is responsible for the cross-border kidnapping of several foreigners in recent weeks.

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. warned the terrorists would likely target places where foreigners gather, such as shopping malls and nightclubs.