Donald Trump has said Germany owes vast sums of money to Nato and the US must be paid more for providing defence.
President Trump reiterated his stance that European allies need to meet their end of the bargain to continue benefiting from the military alliance.
Mr Trump tweeted from his Florida resort the day after his first meeting with Germany's leader which produced some awkward moments, such as not shaking hands during a photo opportunity.
"Despite what you have heard from the FAKE NEWS, I had a GREAT meeting with German chancellor Angela Merkel," the president wrote.
"Nevertheless, Germany owes ... vast sums of money to NATO & the United States must be paid more for the powerful, and very expensive, defence it provides to Germany!"
During a joint news conference, Mr Trump rejected the notion in Europe that his America First agenda means he is an isolationist.
He called such a suggestion "another example of, as you say, fake news," reaffirming the US' "strong support" for Nato, but reiterating his stance that allies need to "pay their fair share" for the cost of defence.
Mr Trump said many countries owe "vast sums of money" but he declined to identify Germany, at the time, as one of those nations.
Prior to his inauguration, Mr Trump declared Nato obsolete but has since modified his stance, telling European leaders the alliance remains of strategic importance.
Only the US and four other members reach the benchmark of spending 2% of GDP on defence. Germany currently spends 1.23% of its GDP on defence but this is being increased.
Mr Trump said the US would do "fantastically well" in its trade relations with Germany.
The president has been deeply critical of foreign trade and national security agreements but suggested he was only trying to revise trade deals to better serve US interests, rather than pull back from the world entirely.
Mr Trump said trade agreements have led to greater trade deficits.
The US trade deficit with Germany was 64.9 billion dollars last year, the lowest since 2009, according to the commerce department.
Mrs Merkel maintained her composure when Mr Trump repeated his contention that former president Barack Obama may have tapped his phones in Trump Tower.
He sought to turn the explosive charge into a light joke when asked about concerns raised by the British Government that the White House is now citing a debunked claim that UK spies snooped on him.
"At least we have something in common, perhaps," said Mr Trump, referring to 2013 reports that the US was monitoring Mrs Merkel's cellphone conversations.
Concerning the most recent report, Mr Trump said he should not be blamed for quoting a Fox News analyst who had accused British intelligence of helping Mr Obama spy on him.
Mrs Merkel attempted conciliation on economic issues.
She said the "success of Germans has always been one where the German success is one side of the coin and the other side of the coin has been European unity and European integration."
Mr Trump backed Britain's departure from the EU and has expressed scepticism of multilateral trade agreements.
The two leaders tried to express their common bonds but showed minimal rapport in their first encounter, a departure from Mrs Merkel's warm relations with Mr Obama during his eight years as president.
Mrs Merkel sought to break the ice, saying that it was "much better to talk to one another than about one another."
She said that while she represents German interests, Mr Trump "stands up for, as is right, American interests. That is our task respectively."
The chancellor said they were "trying to address also those areas where we disagree but tried to bring people together."
"We need to be fair with each other," said Mrs Merkel, saying both countries were expecting "that something good comes out of it for their own people."
The meetings at the White House included discussions on fighting Islamic State, the conflict in Afghanistan and resolving Ukraine's conflict, all matters that require close cooperation between the US and Germany.
The talks aimed to represent a restart of a relationship complicated by Mr Trump's rhetoric on the campaign trail.
As a candidate, Mr Trump frequently accused the chancellor of ruining Germany for allowing an influx of refugees and other migrants from Syria and accused his campaign rival, Hillary Clinton, of wanting to be "America's Angela Merkel."