(CNN) -- Colombian Minister of Defense Rodrigo Rivera resigned Wednesday amid growing concern in the country over increased activity by the leftist Revolutionary Armed Forces of Colombia (FARC).
Rivera's departure marks the first resignation of a cabinet minister in the 13 months that Colombian President Juan Manuel Santos has been in office. Santos praised Rivera's service, and quickly appointed as his replacement Juan Carlos Pinzon, a former vice minister of defense who also has been Santos' chief of staff.
The FARC has been at war with the Colombian government since the 1960s. While severely weakened in recent years, the leftist guerrilla group has continued to carry out kidnappings and attack security forces in the South American nation.
The United States and European Union consider the FARC a terrorist organization.
In the last months local authorities in various parts of Colombia have reported increased FARC activities, targeting mainly civilians and local communities. As many as 40 soldiers and police officers have died this year in attacks attributed to FARC, mainly in the southern part of the country.
Before his departure, Rivera described his last months in office as decisive in the fight against FARC and said, "We have bluntly hit the hardest blows to the organization's drug-trafficking structure in our history."
Pinzon, the new Colombian minister of defense, pledged to continue working with greater strength and decisiveness,and said he would demand from the armed forces an attitude of discipline and ethics.
Journalist Fernando Ramos in Bogota, Colombia, contributed to this report.
