Germany on Monday unveiled plans to buy dozens of US-made F-35 fighter jets, as part of a multi-billion-euro push to modernise its armed forces in response to Russia's invasion of Ukraine.

Berlin intends to buy 35 F-35 jets made by Lockheed Martin to replace Germany's decades-old Tornado fleet, as well as purchasing an additional 15 Eurofighter jets.

Defence Minister Christine Lambrecht called the purchase agreement "a good step forwards" for Germany's Bundeswehr armed forces.

"There can only be one answer to Putin's aggression, and that is unity in NATO and credible deterrence," Lieutenant General Ingo Gerhartz, commander of the German Air Force, told reporters.

Germany's Tornados are the only Luftwaffe planes certified to carry US nuclear bombs stationed in Germany that are a key part of NATO deterrence.

Lockheed's fifth-generation F-35 stealth jets are considered the most modern combat aircraft in the world, and their unique shape and coating make them harder to detect by enemy radar.

The additional Eurofighter jets Germany plans to purchase, made by a consortium that includes Airbus, would be used for other operations, including electronic warfare like jamming enemy air defence systems.

In a landmark speech late last month, German Chancellor Olaf Scholz pledged to invest an extra 100 billion euros ($112 billion) to bring the nation's chronically underfunded Bundeswehr into the modern age.

The spending boost marks a major reversal for Europe's top economy, upending its policy of keeping a low military profile in part out of guilt over World War II.

After years of criticism that the country wasn't shouldering enough of the financial burden in the NATO military alliance, Scholz vowed to spend "more than two percent" of Germany's gross domestic product annually on defence, surpassing NATO's own two-percent target.

The shift was prompted by Russia's invasion of Ukraine on February 24, which shook Germany's sense of security and shone a harsh spotlight on the state of its armed forces.

Germany plans to buy up to 35 F-35 fighter jets made by US firm Lockheed Martin as well as 15 Eurofighter jets
Germany plans to buy up to 35 F-35 fighter jets made by US firm Lockheed Martin as well as 15 Eurofighter jets AFP / JACK GUEZ

The F-35 purchase however raised questions about the future of a common European fighter jet that is being developed with Spain and France.

Known as the Future Combat Air System (FCAS), that plane is slated to replace French-made Rafale jets and German and Spanish Eurofighter planes from 2040.

Scholz sought to allay fears that the project might become unnecessary, saying in February's speech that the joint European project was an "absolute priority".

But the Bundeswehr has to replace its 40-year-old, 93-strong Tornado fleet in the short term because it has become "obsolete", Scholz added.

Lambrecht on Monday also reiterated Germany's commitment to FCAS in the long term.

Lawmaker Johannes Arlt from Scholz's Social Democratic Party (SPD), said the F-35 deal was good news for FCAS in a roundabout way.

"The F-35 builds a bridge to FCAS. The Luftwaffe will learn to fly these fifth generation jets and that will help the FCAS project."

The ultra-modern FCAS jets are considered sixth generation fighters.

Germany's planned jet deal is bad news for US aviation giant Boeing, whose F-18 fighters were considered the frontrunner to succeed the Tornados.

While cheaper than F-35s, the F-18 would have had to have been recertified to be able to transport the atomic warheads hosted by Germany.

The price tag of Germany's new combat jets was not immediately known.

But Finland ordered 64 F-35A jets in December, in a deal worth 8.4 billion euros.

Scholz's centre-left SPD and their coalition partners the Greens have historically been critical of Germany hosting US nuclear bombs, but those voices have faded into the background as the Ukraine war rages.