Apart from Britain, Denmark, Portugal, Ireland and Malta have yet to decide whether to join the pact.
The United States warned European members of North Atlantic Treaty Organisation (Nato) earlier this year to boost their defence spending or risk the scaling down of Washington's commitment to the alliance.
PESCO obligates European Union member states to cooperate more and more systematically: increasing defense spending, making their units available for European Union operations, creating common military powers and strengthening the defense industry.
The European Council has agreed in June to establish PESCO and the EU leaders discussed the progress in preparing during last summit.
Membership of the Permanent Structured Cooperation (PESCO) initiative will present an opportunity for Slovakia to bolster its defence capabilities, both for national purposes and in meeting its global commitments, said Defence Minister Peter Gajdoš (a Slovak National Party/SNS nominee) who signed an intention for Slovakia to join PESCO along with State Secretary for Foreign and European Affairs, Ivan Korčok, in Brussels on November 13.
"The real problem is not how much we spend, it is the fact that we spend in a fragmented manner", Mogherini said.More news: Pixel 2 problems surface again, this time with Google Assistant
Although the initiative, known as Permanent Structured Cooperation (PESCO) in European Union jargon, is a Franco-German brainchild, some differences between Berlin and Paris have emerged, diplomats said.
"I'm a firm believer of stronger European defense, so I welcome PESCO because I believe that it can strengthen European defense, which is good for Europe but also good for NATO", Stoltenberg said. The move, driven by France and Germany, is misguided.
The notification letter - co-authored by France, Germany, Italy, and Spain - described the pact as "an ambitious, binding, and inclusive European legal framework for investment in the security and defense of the EU's territory and its citizens".
Pesco could, in theory, lead to the creation of a European operational headquarters or logistics base, but will first focus on projects to develop new military equipment such as tanks or drones.
The agreement also commits countries to "regularly increasing defense budgets in real terms" as well as devoting 20 percent of defense spending to procurement and two percent to research and technology.