NATO's secretary-general says a core version of the alliance's new agile expeditionary force is now operational.

Jens Stoltenberg told reporters in Berlin on Wednesday that the interim force is made up of troops from Germany, Norway, the Netherlands and "some others."

He says "an interim solution is in place and we are focused on a more permanent high-readiness force."

Stoltenberg says it's too early to say when the full permanent force will be in place or how it will be paid for, though it will have to be funded through a combination of NATO's budget and individual nations' contributions.

The "spearhead" force, conceived to be sent at short notice to aid at-risk members, was approved by NATO leaders in September amid concerns over Russia's actions in Ukraine.