Two ground-to-air missiles have been found in arms caches uncovered in raids against suspected members of Basque separatist group Eta, officials said.
Suspected leaders Albizu and Iparraguirre were seized on Sunday
The missiles were found with a large amount of explosives in south-west France, near the Spanish border, French and Spanish officials said.
On Sunday, a joint France-Spain police operation led to the arrest of two suspected leaders and group members.
Spain vowed to crush the group after the raids.
The missiles found are Russian-made, "in perfect condition and ready to be used", police said.
The SAM-7 shoulder-fired missiles were recovered on Monday night from underground caches in houses in Urrugne and Briscous, they said.
Suspected Eta leader Mikel Albizu, also known as Mikel Antza, was among those arrested at the weekend, along with his girlfriend, Maria Soledad Iparraguirre.
She is also thought to be one of Eta's most senior figures and is wanted by the Spanish authorities for her suspected involvement in at least 14 assassinations.
Their arrests were part of a series made in border villages along the French border, where Eta militants have long sought refuge.
Officials said Sunday's operation was possibly the most significant blow against Eta in years.
The armed group has been fighting for more than 30 years for an independent Basque state.