"The monarch, who travels with Defense Minister Jose Antonio Alonso, arrived at a base in [the western city of] Herat early in the morning to meet with Spanish troops and study the situation in the country," the radio said.
The king's visit has not been announced for security reasons, the radio station said.
Spain has 742 troops in the Herat and Badghis provinces deployed there as part of the UN-led peacekeeping mission. The Spaniards mainly provide logistic support in operations against mujahidin and train Afghan military personnel.
Spanish troops were first deployed in the war-torn country in 2002. Their death toll has since reached 86 people, including 62 soldiers who died in May 2003 in an air crash in Turkey.
Afghanistan, where the Taliban regime was overthrown in the 2001 U.S.-led campaign, has seen a rise in violence in the last two years. The country is also the world's leader in opium trade, with almost all its arable lands being sown with poppy.
