Officials fear a wave of explosions in Kenya could indicate another large-scale attack is coming.

Since the September attack on an upscale mall that killed 67 people, Somali militant sympathizers have been blamed for an explosion at Nairobi's main airport, a grenade attack on tourists on Kenya's coast, a blast on a public bus in Nairobi, and three blasts Monday night in the capital that killed six people.

A top police official who insisted on anonymity said security agencies believe a large attack is imminent. Police foiled a car bomb attack in Mombasa last month.

Australia issued a weekend alert of "a serious and ongoing risk of large scale acts of terrorism" in Kenya. The U.S. Embassy last month reminded its citizens of the threat of terrorism.