The German military is to invest billions of euros to fix recurring problems with its aircraft, helicopters and other military equipment, an official said on Friday.

"We will invest up to 6 billion euros ($6.5 billion) in the next seven to eight years to obtain improvements in these areas," a defence ministry spokesman told AFP.

Der Spiegel news weekly, which first revealed the investment programme, said the army planned 120 measures to boost personnel and stocks of spare parts.

The paper Der Spiegel cited "numerous" defence documents, which confirmed the "hardware problems constraining the capacity" of the Bundeswehr.

It listed radar problems with Eurofighter jets and with winches on board the NH90 transport helicopters that hampered their use in operations.

Only four of military's 39 NH90 helicopters are currently useable, Der Spiegel said.

Germany's military has come in for repeated criticism in recent months over the state of its weaponry.

The G36 assault rifle became the butt of jokes following reports it had trouble firing straight in high temperatures.

The army has since stop using the rifle.

In September, Defence Minister Ursula Von der Leyen admitted that the army could not currently meet its commitments to NATO in the event of an attack on an alliance member.

She blamed the situation partly on delays in the supply of spare parts for planes and helicopters.