Colombian President Juan Manuel Santos has replaced his defense minister amid rising concerns over a surge in guerilla attacks.

Rodrigo Rivera resigned on Wednesday, saying he was going to “explore new opportunities.”

Mr. Santos praised Rivera's accomplishments at the news conference announcing his departure. But both Mr. Santos and Rivera have faced growing criticism over the upswing in violence since Mr. Santos took office last year, including a sharp increase in kidnappings in the first six months of 2011 compared to the same period a year earlier.

Succeeding Rivera is Juan Carlos Pinzon, a top aide to the president. Pinzon is from a traditional military family and has also previously served as deputy defense minister.

President Santos has appointed Rivera to an ambassadorship in Brussels.

Colombia has been fighting against rebels from the FARC, or Revolutionary Armed Forces of Colombia, since the 1960's. The FARC has been designated as a terrorist organization by Colombia, the United States and the European Union.