Two small bombs have exploded in the historic Colombian city of Cartagena, just hours after US President Barack Obama arrived for a summit there.
The devices went off near a bus terminal on Friday evening.
Two further bombs exploded in the capital Bogota, close to the American embassy.
Police say they may have been planted by leftist rebels.
Barack Obama is among 30 heads of state who have travelled to Colombia for the Summit of the Americas.
US influence has waned in Latin America in recent years.
China has come out on top in terms of trade, becoming the number one partner for countries like Brazil.