Germany's Greens may refuse to back plans by likely next chancellor Friedrich Merz for a massive increase in state borrowing, with a party source on Monday warning "approval is becoming less likely with each passing day".

Germany's Greens vowed to block plans by likely next chancellor Friedrich Merz for a massive increase in state borrowing but left room for compromise on Monday on measures to revamp the military and revive growth in Europe's biggest economy.

German election winner Friedrich Merz's conservatives and the Social Democrats (SPD) have concluded preliminary talks towards forming a coalition government, heads of the parties said on Saturday.

German Chancellor-in-waiting Friedrich Merz is open to making concessions to the Greens to secure support for constitutional changes that would unleash hundreds of billions of euros in defense and infrastructure spending.