European Union countries have officially launched a new era in defense cooperation with a program of joint military investment and project development to help the EU confront its security challenges.

Twenty-three of the EU's 28 member states signed up Monday to the program, known as permanent structured cooperation, or PESCO. Britain, which is leaving the EU in 2019, and Denmark with a defense opt-out were among those not taking part.

EU foreign policy chief Federica Mogherini said countries have already submitted more than 50 joint projects in the fields of defense capabilities and military operations.

She said PESCO, backed by the EU defense fund, "will enable member states to use the economy of scale of Europe and in this manner to fulfil the gap of output that we have."