A statement from the Spanish police said the uniforms would have been "enough to equip an entire army".
ISIS's uniforms may be camouflage, but the pattern did not help hide them from authorities who confiscated an army's worth of clothing being smuggled through Spain.
Up to 20,000 military uniforms hidden inside aid packages have been uncovered after Spanish authorities busted a large scale smuggling ring to jihadi groups in Syria and Iraq.
The National Police discovered three containers of goods in a port in Valencia and one in Algeciras.
"With the roughly 20,000 military uniforms and accessories, it would have been possible for [this army] to be ready to enter into combat in any of the battlegrounds where jihadist terrorist organisations are operating around the world".
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Fadi al-Halabi/AFP/Getty Images Terror groups like the Islamic State group and the Nusra Front receive support for their operations in foreign countries like Spain.
The equipment was shipped in sealed containers under cover of humanitarian aid and remittances were funded by the "hawala", an informal Islamic money transfer system, much more discreet than traditional bank transfers.
The National Police said last month in a statement that the leader of the network was reportedly in "constant contact" with a member of the ISIS terror group, who repeatedly requested him to recruit women, to be married off to ISIS jihadists in Syria.
The New York Times reported that five of the seven people were Spanish citizens. Police said the suspects, arrested in the cities of Alicante, Valencia and Ceuta, were part of a criminal network that had made multiple shipments to Syria and Iraq while posing as a humanitarian organization.