Likely next chancellor Friedrich Merz said Tuesday he planned to reform the country's strict constitutional "debt brake" to pump up defence spending and pursue massive infrastructure investments.

The announcement came just over a week after national elections and with talks underway between Merz's conservative CDU/CSU bloc and the centre-left Social Democrats to forge a new governing alliance.

The two sides however agreed to try to rush through the changes before a government is formed, as US President Donald Trump's public falling out last week with Ukrainian counterpart Volodymyr Zelensky added to the sense of urgency in Europe.

Trump's moves to open direct talks with Russian President Vladimir Putin have left Europeans feeling their interests could be overlooked in any deal to end the fighting that started with Moscow's full-scale invasion three years ago.

Germany's EU partners, who are gathering for a key summit on Thursday to discuss Ukraine and defence, have been waiting for movement from Berlin after months of political paralysis since the collapse of Chancellor Olaf Scholz's government in November.

- 'Dizzying' -

The proposed reform of Germany's budget rules would see military expenditure over one percent of GDP exempted from constitutional limits on taking on new debt.

In practice, the reform could see Germany's defence budget double to 100 billion euros ($107 billion) per year, said Manuela Schwesig, a senior Social Democrat involved in coalition talks.

CDU leader Merz and his partners also want to set up a 500-billion-euro fund to upgrade Germany's creaking infrastructure and help drag the country out of two straight years of recession.

The size of the investments in consideration were "dizzying", Markus Soeder, the leader of the CDU's Bavarian sister party the CSU, said Wednesday.

But the announcement sent a clear signal: "Germany is defending itself, Germany is arming itself, Germany is renewing itself."

After coming out on top in last month's vote, Merz said he had "no illusions" about the US president and has wasted no time in ditching his party's traditional aversion to debt to boost Germany's defences.

The constitutional change is set to be rushed through the outgoing parliament, where the conservatives and the Social Democrats would have the necessary two-thirds majority with the Greens.

According to the Frankfurter Allgemeine Zeitung newspaper, the first debate on the reform is planned to take place in the Bundestag on March 13, with the final vote four days later.

If the plan succeeds, it could be a "real game changer" and "quickly overcome Germany's economic stagnation", said Sebastian Dullien, director of the IMK economic research institute.

- European coordination -

Merz met on Wednesday with current Social Democrat Chancellor Olaf Scholz, who welcomed the moves to boost defence, according to his spokesman Steffen Hebestreit.

The conservative leader also travelled to Brussels on Wednesday, where he held talks with NATO chief Mark Rutte.

Merz posted on X that the talks centred on "the defence capabilities of Europe and NATO and challenges related to security policy."

"Germany and Europe must now invest massively in their own defence capabilities. That is the best protection for Europe's security."

Jacob Ross of the German Council on Foreign Relations said the importance of Merz's plan has "a lot to do with psychology and the signals that such announcements send".

But he said there is also the question of coordination with other EU states and with Brussels, where commission chief Ursula von der Leyen also presented a plan on Tuesday to mobilise some 800 billion euros for Europe's defence and support for Ukraine.