Brexit and doubts about US President Donald Trump's commitment to European security have given fresh impetus to EU ambitions to step up military cooperation, with France and Germany taking the lead.
"This is a big leap forward. It is supposed to start working initially in March and fully in June," one of the sources said.
EU foreign and defence ministers meeting Monday in Brussels will approve what is known as a Military Planning and Conduct Capability (MPCC) unit, the sources said.
The EU also runs Operation Sophia in the central Mediterranean, which can use force to stop migrant smugglers, and Atalanta, part of international anti-piracy forces off the Horn of Africa.
More From This Section
These "executive" operations have their own command centres which will remain separate.
The diplomatic sources said the MPCC would show the EU was prepared to do more to contribute to international security but also stressed its limited role, at least for now.
European Commission head Jean-Claude Juncker called for a common EU defence HQ in September after the Brexit vote, resurrecting an idea that had circulated in the EU for years.
But some EU nations such as Britain and the former communist states in eastern Europe, are reluctant to do anything that might undercut NATO's collective defence role, believing the US-led alliance is still the only real option in face of a more assertive Russia.
The MPCC, based in Brussels, will have a small staff and be headed by a "director" not a commander, to avoid overt military connotations, another EU source said.
"The general intention is to explore ways of how to use our capabilities more effectively," the source added.