Colombia's defense minister abruptly resigned Wednesday amid a resurgence of violence by rebel and criminal gangs that has reversed some of the security gains made over the last decade.

Recent attacks on the country's energy infrastructure and kidnappings of oil company personnel are especially worrisome to business interests concerned that continued deterioration could jeopardize an ongoing oil boom.

In a letter to President Juan Manuel Santos, outgoing Defense Minister Rodrigo Rivera said it was time to "put an end to this chapter of my life and explore other opportunities."

At a subsequent news conference that Rivera attended, Santos praised Rivera for his yearlong tenure directing the country's police as well as armed forces. During that time, Santos said, 2,400 guerrillas were "neutralized," meaning either killed in action or demobilized.

But there is a widespread perception, underscored by recent opinion polls, that violence has increased both in urban centers and in the countryside. Some statistics bear out that view, particularly those related to leftist rebel activity. Other data, including those showing no shift in the total number of homicides, indicate little change since Santos took over from former President Alvaro Uribe in August 2010.

Energy companies would probably agree that conditions have worsened. Kidnappings in Colombia so far this year total 193, up 25% from the same period last year, according to Bogota-based security consultant Orlando Hernandez. Those seizures include 38 oil field personnel, many of whom are still missing.

Attacks on electrical towers this year total 36, more than double the 14 such assaults over the same period last year. Bombings of pipelines and other oil infrastructure are at 13, up from 11 last year. This month, guerrillas struck an Ecopetrol crude storage facility in North Santander state, destroying several tanks.

Industry officials have warned that violence must be curbed or Colombia could lose much of the foreign investment now flooding into the country, the bulk of it directed toward petroleum and coal mining. The arrival of foreign wildcatters has pushed oil output to 950,000 barrels a day on average.

Earlier this week, Santos announced new measures to tighten highway security in southeastern Caqueta state, the scene of several attacks by the leftist Revolutionary Armed Forces of Colombia against oil company personnel exploring for heavy oil.

Santos named his advisor and former defense vice minister Juan Carlos Pinzon to replace Rivera as defense minister.

Rivera was not out of a government job: At the news conference, Santos named him Colombia's new ambassador to the European Union in Brussels.

Kraul is a special correspondent.