La Paz, Feb 19 (IANS/EFE): The Bolivian government Monday nationalized the Spanish-owned firm that runs the Andean nation's three largest airports, accusing the company of not investing enough in improvements.

President Evo Morales announced the move in the central city of Cochabamba, where Servicios de Aeropuertos Bolivianos S.A. is headquartered.

Bolivian military units were deployed to the three airports "to guarantee continuity of service", the leftist head of state said.

Sabsa, as the company is known, becomes the sixth Spanish-owned Bolivian enterprise to be nationalized in less than a year.

A 25-year-concession to operate Bolivia's three main airports - serving La Paz, Cochabamba and Santa Cruz - was awarded to Sabsa in 1997.

Sabsa has changed hands twice since then, passing in 1999 from US-based Airport Group International to Britain's TBI, which five years later sold the firm to two Spanish concerns: state-owned aviation administrator AENA and infrastructure giant Abertis.

Sabsa says it has spent $33.6 million on improvements to the three airports during the life of the concession, but Public Works Minister Vladimir Sanchez said last week that the concessionaire has invested less than $1 million annually since 2005.

Bolivia will appoint an independent auditor to determine appropriate compensation for Abertis and AENA.

La Paz's nationalization of Sabsa "will have consequence for bilateral relations", Spanish Foreign Minister Jose Manuel Garcia-Margallo said.