French police have found two Russian-made surface-to-air missiles among huge arms caches uncovered during a sweep against Basque separatist guerrilla group ETA in south-western France, officials said on Tuesday.
French police sources said they were SAM-7 missiles that can be used to bring down helicopters or low-flying aircraft.
The Russian missiles were "in perfect condition and ready to be used", Spain's Interior Ministry said in a statement. It did not identify the type of missile.
Police found several ETA arms caches in the French Basque country on Sunday when police in France and Spain arrested 21 ETA suspects - including the group's alleged leader Mikel Antza - in the biggest police coup against ETA in 12 years.
The missiles, together with stockpiles of assault weapons, grenade launchers and more than a tonne of explosives, indicate that ETA had more sophisticated and powerful weaponry than previously thought.
Although ETA has not staged a fatal attack in 16 months and has only carried out small-scale bombings recently, it apparently has the capability for more serious attacks.
Newspaper El Pais said on Tuesday the operation had dismantled ETA's "strategic arms reserve".
Spanish media reports said the missiles were similar to one found in an ETA cache in 1986. ETA is not known to have attempted to use a missile to bring down an aircraft.
ETA, classed as a terrorist group by the European Union and the United States, has killed more than 800 people since 1968 to press its demand for a Basque state carved out of northern Spain and south-western France.
A detailed list of weapons found so far included 70 grenades, 25 cases of Titadyne dynamite, timers, pistols, 30 assault rifles, 50,000 euros in cash, 500 detonators, 44 sub-machine guns, 36,000 cartridges and 31 grenade launchers, Spain's Interior Ministry said.
Police have also found an underground shooting gallery and seized documents.
Police believe all of the weapons were acquired in the last five years, the Spanish Interior Ministry said.
"The French police estimates that ETA has invested the greater part of its funds over this period in acquiring this material on the black market," it added.
It said police were hunting for more hidden caches that might have hydraulic opening systems and expected to find more weapons.
French and Spanish police have arrested hundreds of ETA suspects in recent years, severely weakening the organisation.
Three of those detained by French police on Sunday have now been released.