The Spanish authorities say they have arrested five suspected members of Basque separatist group Eta who were planning to carry out attacks.
Evidence from earlier arrests is said to be behind the latest detentions
The four men and a woman were detained during raids in the Basque province of Guipuzcoa and the neighbouring region of Navarra.
Police say they were acting on information following the weekend arrests of 20 Eta suspects in France.
Two ground-to-air missiles were found in further operations on Monday.
Speaking after the latest arrests, Interior Minister Jose Antonio Alonso said: "The Civil Guard believes that they had agreed to join the operational structure of the terrorist organization Eta."
The ministry said the arrests were made possible by statements from a Spanish truck driver arrested in northern Spain on Sunday.
Spanish officials have described the recent arrests and discovery of arms caches as possibly the most significant blow against Eta in years.
The weekend detention of suspected Eta leader Mikel Albizu, also known as Mikel Antza, and his girlfriend, Maria Soledad Iparraguirre was a particular coup for the authorities.
Ms Iparraguirre is 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.
Suspected leaders Albizu and Iparraguirre were seized on Sunday
The couple and nine others were transferred to Paris for questioning.
The Russian-made SAM-7 shoulder-fired missiles, recovered on Monday night from underground caches in houses in Urrugne and Briscous, were said to be in "in perfect condition and ready to be used".
Mr Alonso said: "We don't have any evidence they were going to be used in any specific operation, but the mere fact they had them is alarming news."
Eta has been fighting for more than 30 years for an independent Basque state.