New Delhi, April 29 -- A spate of attacks against civilians and military bases in Colombia's southwestern region has raised security concerns as the country heads to a May presidential election in which crime is expected to be one of the top voter concerns.

Rebel groups have staged 26 attacks with explosives and drones since Friday, including a deadly blast Saturday on a highway between the cities of Cali and Popayan, according to Colombia's defense ministry. The death toll in that explosion rose to 21 people on Monday.

Violence in the region is nothing new. Illegal groups have sought to control the area for decades, deeming it strategic for illicit activities, such as illegal mining and drug trafficking, including the cultivation of co...