Any such attack in Nairobi would immediately arouse suspicion that Somali militants from al-Shabab carried out the assault. The al-Qaida-linked group earlier this month promised to unleash terror attacks in Nairobi in response to a push by Kenya's military into southern Somalia.
Monday's grenade explosion, though, did not bear the hallmarks of well-planned terror attack. The U.S. on Saturday warned of an imminent terrorist attack and said likely targets include shopping malls and night clubs where foreigners congregate. Conversely, the grenade explosion occurred at a downtrodden bar where only lower-income Kenyans meet.
Police Commissioner Mathew Iteere told a news conference Monday that the grenade exploded at 1:15 a.m. while 20 people were inside the bar. Three people were seriously wounded, he said. The blast overturned chairs and tables, and blood stained the floor. Iteere said police did not yet have any suspects.
"It is too early at this point in time to give a conclusive answer," he said, adding later: "The person who lobbed the grenade into the pub was not seen by anybody."
Police have tightened security around hotels, bridges and fuel depots, Iteere said.
The weapon used Monday was a Russian-made F1 grenade, he said. A similar type of grenade was used in a downtown Nairobi attack in December 2010 at a bus station. That attack killed one person.
Three grenades exploded at a political rally in downtown Nairobi in June 2010, killing six people. In December that year two traffic police died when they were shot and a third was seriously injured by a grenade.