Top officials from NATO and the European Union pledged Friday to step up their cooperation, which has proven challenging in the past because of long-standing tensions between NATO member Turkey and EU member Cyprus.
"Even though our internal and external security are closely linked, sometimes it seems as if the EU and NATO were on two different planets, and not headquartered in the same city," EU President Donald Tusk said. "It makes sense to work more closely together."
The animosity between Cyprus and Turkey has deep historic roots. Cyprus has been split since Turkish troops invaded the northern third of the island in 1974, in response to a Greek-inspired coup aimed at uniting the island with Greece.
But the revival of peace talks aimed at reunifying the Mediterranean island has helped allay tensions between Ankara and Nicosia.
Cooperation between Greece and Turkey through a NATO mission cracking down on migrant smuggling in the Mediterranean also helped pave the way to the pledge made on Friday, EU sources said.
"We have actually concluded more formal arrangements in the past six months than in the previous 13 years," NATO chief Jens Stoltenberg said on Friday, after signing a declaration with Tusk and European Commission Jean-Claude Juncker in Warsaw.
The two EU officials were later due to take part in a NATO summit being held in the Polish capital.
The declaration calls for a "new impetus and new substance to the NATO-EU strategic partnership."
The two sides are for instance keen to cooperate on cybersecurity, capacity building in third countries and hybrid threats - an approach most recently used by Russia, consisting of covert military and intelligence operations while mostly avoiding direct combat.
NATO is also expected to offer support to the EU's Operation Sophia, a naval mission that was set up last year to patrol international waters off Libya and arrest migrant smugglers.
"Our security is interconnected and we face a range of unprecedented challenges. Separately, neither NATO nor the EU can fully address them. But together, we are a formidable team," Stoltenberg said.