"The war in Ukraine has opened the eyes of European society, including in Spain," said Sanchez at a commemorative event at Madrid's Teatro Real attended by King Felipe VI and NATO Secretary General Jens Stoltenberg.

"Many have understood that our security is no longer guaranteed," he added.

"Today, our security is under threat from the regime of (Russian President Vladimir) Putin," Sanchez said, making it "essential" that "we reinforce our deterrence capabilities".

That would "require military capabilities that were modern, viable, and deployable, which can only be acquired through increased investment in defence," he said.

"The cost of doing nothing would be far higher," he added.

Spain invested 1.03 percent of its gross domestic product on defence spending in 2021, one of the lowest figures among members of the North Atlantic Treaty Organization. Only Luxembourg invested less, according to data from the military alliance.

Sanchez pledged to increase Spain's investment to the 2.0 percent required by the alliance, but he is likely to face opposition from his hard-left junior coalition partner Podemos, which refused to join Monday's event.

On June 29 and 30, Madrid will host the NATO summit, which comes at a crucial time following Russia's invasion of Ukraine and the resulting bid by Sweden and Finland to join the alliance, ending decades of military non-alignment.

Their request to join NATO is one of the most significant changes in Europe's security architecture in decades, not least because Finland shares a 1,300-kilometre (800-mile) border with Russia.

Stoltenberg said it would be "a historic summit".

"At the Madrid summit, we will chart the way ahead for the next decade," he said, referring to plans to revise and update NATO's strategy, which was last reset in 2010.