Taking up 30,000 square metres -- the size of five football pitches -- the factory in Unterluess in northern Germany will be able to produce 350,000 artillery shells a year by 2027.

"Building this factory proves that Germany can do things quickly when it really matters," Rheinmetall chief Armin Papperger said according to a manuscript of his speech.

"Our country -- as part of a Europe which has to take responsiblity for its security now more than ever -- is taking responsibility."

NATO chief Mark Rutte was attending the plant's opening ceremony alongside German Defence Minister Boris Pistorius and Finance Minister Lars Klingbeil.

Berlin has moved to rearm itself following Russia's full-scale invasion of Ukraine in 2022, accelerating that process as US President Donald Trump urged Europe to take more responsibility for its own defence.

Chancellor Friedrich Merz has vowed to build Europe's "strongest conventional army" and has amended debt rules to let the government borrow billions for military equipment.

Papperger said the plant's construction over the past 15 months was a "direct consequence" of Merz's predecessor, Olaf Scholz, announcing a "turning point" in German defence policy following Russia's full-scale invasion of Ukraine.

The plant will help fill a record-breaking munitions order worth 8.5 billion euros (then $9.3 billion) placed by the German government in July 2024.

Rheinmetall's Unterluess site already makes guns and munitions for the Leopard 2 tank, which has been used by the Ukrainian army.

Germany's cabinet signed off a draft law earlier on Wednesday that aims to boost armed forces recruitment and includes provisions for compulsory military service if there aren't enough volunteers.

About 182,000 soldiers currently serve in the armed forces. Pistorius has said that should rise to 260,000.

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RHEINMETALL