Donald Trump says Germany owes "vast sums of money" to NATO, just a day after meeting Angela Merkel at the White House.
The president said the US must be paid more for providing defence in a tweet he posted while at his weekend retreat in Florida.
He reiterated his stance that Germany needs to meet its end of the bargain if it is to continue benefiting from the military alliance.
Mr Trump wrote: "Despite what you have heard from the FAKE NEWS, I had a GREAT meeting with German Chancellor Angela Merkel."
He added: "Nevertheless, Germany owes ... vast sums of money to NATO & the United States must be paid more for the powerful, and very expensive, defense it provides to Germany!"
But a former US ambassador pointed out that NATO members don't pay the United States for security, but contribute by spending on their own militaries.
Ivo Daalder tweeted: "Sorry, Mr President, that's not how NATO works.
"This is not a financial transaction, where NATO countries pay the US to defend them. It is part of our treaty commitment."
Mrs Merkel and Mr Trump appeared to have an awkward meeting at the White House, with the US President seemingly refusing to shake the German Chancellor's hand in front of the media in the Oval Office.
They had, however, shaken hands when they met earlier.
The two leaders discussed NATO funding, trade agreements and the international situations in Ukraine and Syria.
During their joint news conference on Friday, Mr Trump, who has dubbed NATO "obsolete", demanded America's allies pay back the "vast sums of money from past years".
Mrs Merkel said Germany had committed to increasing its military spending to the previously agreed level of 2% of GDP for NATO's European members.
Besides the United States (at 3.36% of GDP), only Britain (2.17%), Poland (2.01%), Estonia (2.18%) and Greece (2.36%) currently reach the goal, according to NATO estimates for 2016.
Mr Trump set the tone for his relations with the German chancellor during his campaign last year, saying her decision to allow refugees into Germany was a "catastrophic mistake" and suggested she was "ruining Germany".