European Union leaders and NATO's secretary-general are meeting to discuss how their organizations can better cooperate to face new challenges to European security, including from Russia and from Islamic radicalism in North Africa and the Middle East.

NATO secretary-general Jens Stoltenberg said Friday that "a strong Europe makes also NATO strong."

Of the 28 EU member countries, 22 also belong to the U.S.-led NATO political and military alliance.

NATO and the EU, both based in Brussels, have increasingly been coordinating their actions, including how to deal with Russia following its annexation of Crimea from Ukraine. Instability in Europe's south caused by Muslim extremism is also a joint concern.

Stoltenberg told reporters that "both NATO and the European Union have to adapt."