French President Emmanuel Macron and German Chancellor Angela Merkel have pledged to work closely together to draw up a "road map" of reforms for the European Union.
The pair said they are both prepared even to implement treaty changes if needed.
The two met in Berlin on Mr Macron's first full day in office, which started with him naming 46-year-old Edouard Philippe as his new prime minister.
Mrs Merkel called Mr Macron's visit an honour and a sign of the deep friendship between the two European powerhouses.
"We each represent the interests of our own countries, but the interests of Germany are naturally closely tied to the interests of France," Mrs Merkel said.
She added that she hoped for a "new dynamism" in German-French relations, saying that "Europe will only do well if there is a strong France, and I am committed to that".
Mr Macron said that he would work with Mrs Merkel on a "road map", and that they needed to work on "deep reforms that are necessary and need common work".
Both leaders suggested they were prepared to change European treaties if needed, but Mrs Merkel stressed that such measures were not immediately on the table.
"First we need to work on what we want to change, and then if it turns out it needs a treaty change, then we're prepared to do that," she said.
The visit to Berlin continued a tradition of French presidents making their first foreign trip to Germany.
A large group of onlookers, some carrying European flags, stood outside the chancellery as Mr Macron arrived.
Germany and France have traditionally been the motor of European integration, but the relationship has become increasingly lopsided over recent years as France struggled economically.
The visit signalled Mr Macron's intentions to move rapidly on campaign promises to revive support for the beleaguered European Union by reforming and strengthening it.
In a nod to German concerns, Mr Macron said at the news conference with Mrs Merkel that he had never pushed for jointly issued eurobonds and does not favour European countries taking joint responsibility for old debts.
Germany, which has Europe's biggest economy, has vehemently opposed taking direct responsibility for weaker eurozone countries' debts.
Mr Macron added, however, that "what I know is that we have investments to make (in Europe), and so we have to work on investment mechanisms for the future".
Earlier, the appointment of Mr Philippe to the top job in his government ticked several boxes for Mr Macron, at 39 France's youngest president, who took power on Sunday.
Mr Philippe's age reinforced the generational shift in France's corridors of power and the image of youthful vigour that Mr Macron is cultivating.
He is also relatively unknown to voters, fulfilling Mr Macron's campaign promise to repopulate French politics with new faces.
Mr Philippe is the mayor of the Normandy port of Le Havre, a trained lawyer and an author of political thrillers.
His appointment marks a milestone in the rebuilding of France's political landscape, which has been ignited by the election of Mr Macron - the first president of modern France not from the country's mainstream left or right parties.
Mr Philippe is a member of the mainstream-right Republicans party and could possibly attract other Republicans to Mr Macron's cause, as the centrist president works to piece together a majority in parliament to pass his promised economic reforms.
Alain Juppe, a former French prime minister, called Mr Philippe "a man of great talent" with "all the qualities to handle the difficult job".
In Berlin, Mr Macron declined to answer a question about his new prime minister, only saying the choice of Mr Philippe is part of the new political landscape he is promoting.
Speed is becoming one of Mr Macron's trademarks - the announcement of Mr Philippe's appointment delivered by the presidency's new secretary general, including the "thank you" at the end, took just eight seconds.
As well as the political coup of poaching Mr Philippe from the right, Mr Macron is also siphoning off support from politicians on the left.
At least 24 Socialists are now campaigning for re-election under the banner of Mr Macron's Republic on the Move party.
But not everyone was pleased with the Philippe announcement.
For far-right leader Marine Le Pen, Mr Macron's rival for the presidency, the selection of Mr Philippe reflects a continuation of the system she hoped to break.
"This is the sacred alliance of the old right and left, united in their wish to remain in place at any price," Ms Le Pen said in a statement.
Defeated by a Macron landslide, she denounced what she predicted would be a continuation of old policies, including "austerity, submission to Brussels, massive immigration".
The populist Ms Le Pen said her National Front party is now the only "true opposition" for June legislative elections.
French far-left leader Jean-Luc Melenchon, who won nearly 20% of the vote in the first round of the presidential election, also reacted with hostility.
Voters go to the polls again in June to elect 577 National Assembly politicians.
Mr Melenchon urged them not to give Mr Macron a parliamentary majority.
"The right has just been annexed, with a prime minister taken from its ranks, from the Republicans," Mr Melenchon said.
"Don't give full powers to Mr Macron and his prime minister."
Mr Macron's trip to Berlin highlighted his pro-European politics and desire to work with Mrs Merkel on what he says must become "a more efficient Europe, a more democratic Europe, a more political Europe".
Mr Macron previously met Mrs Merkel when he visited Berlin in March as a candidate.
Germany is looking to Mr Macron to revitalise France as an economic power and political heavyweight in the EU, which is facing complex divorce proceedings with its current number two economy, Britain.
When Britain leaves the bloc in 2019, France will be the EU's only member with nuclear weapons and a permanent seat on the UN Security Council.
As a candidate, Mr Macron called for a "new Franco-German deal" that would involve "much more structured co-operation" on investment, on European border security and on defence.
Mr Macron is the conservative Mrs Merkel's fourth French president in nearly 12 years as chancellor.
She built a solid relationship with Mr Macron's predecessor, Socialist Francois Hollande, despite their political differences - notably with their joint effort to secure an accord to calm the fighting in eastern Ukraine in tense talks in Minsk, Belarus, in 2015.
Germany is keen to continue the Franco-German diplomatic drive to keep a lid on the situation in Ukraine, where Russia-backed separatists are battling the government.
Mrs Merkel has praised Mr Macron's embrace of European unity but has offered few concrete details about the way forward for German-French relations.