Madrid (AFP) - Spanish police said Thursday they have seized about 20,000 military uniforms, "enough to equip an entire army", which were destined for jihadist groups operating in Syria and Iraq.

The uniforms were found in three shipping containers seized in the eastern ports of Valencia and Alicante last month when police uncovered an operation to smuggle arms to jihadists under the guise of humanitarian aid, police said in a statement.

Seven people were arrested at the time as part of a probe launched in 2014 into "foreign structures" providing logistical support for both Islamic State and the Al-Nusra Front, an Al-Qaeda affiliate in Syria.

"The containers which carried the military uniforms were declared as 'second hand clothes' so as to not raise suspicions and be able to pass different customs inspections without any difficulty," the police statement said.

"With the roughly 20,000 military uniforms and accessories, it would have been possible to equip an entire army which would be ready to enter into combat in any of the battlegrounds which jihadist terrorist organisations have round the world," it added.

One of the firms run by the suspects who were detained last month was dedicated to importing used clothes.

One of those arrested was a man who dispatched "military material, money, electronic and transmission material, firearms and precursors for making explosives" to Syria and Iraq via a company, police said at the time.

This was shipped out in closed containers under the guise of humanitarian aid, and financed by "hawala," an informal system of payment based on trust that is far more difficult to trace than bank transfers.

The leader of the network was in "constant" contact with a member of the Islamic State, who repeatedly asked him to recruit women in order to marry them off to IS jihadists in Syria, police said last month .