German Chancellor Friedrich Merz warned Monday that Russian President Vladimir Putin's "imperialist plan wouldn't end with the conquest of Ukraine but would rather be just the start".

Merz told a conference of Germany's ambassadors that "we are experiencing daily and with increasing intensity hybrid Russian attacks, including on our infrastructure" and pointed to Moscow's "provocations in the North and Baltic Seas".

Germany has been Ukraine's second-biggest supplier of military aid since Russia's invasion began in 2022 and has been on high alert for sabotage plots directed from Moscow.

Merz has moved to ramp up Germany's defence capacities in the face of US President Donald Trump's questioning of the future strength of the transatlantic alliance and wants Germany to have Europe's "strongest conventional army".

"We have historic tasks," Merz said on Monday, namely "building a new security architecture which should last for several decades to come".

"What we referred to as the liberal world order is under pressure from many sides, including within the political West," he said.

"A new conflict between systems has broken out between liberal democracies and an axis of autocracies."