US President Donald Trump has reportedly called on German Chancellor Angela Merkel to pay hundreds of billions of dollars Berlin owes to the North Atlantic Treaty Organization (NATO), a German minister says, calling the request “outrageous.”

Trump handed the bill – thought to be for more than £300 billionns (US$375 billions) —to Merkel during their last week meeting in Washington, DC, the Sunday Times reported, citing an unnamed German minister.

“The concept behind putting out such demands is to intimidate the other side, but the chancellor took it calmly and will not respond to such provocations,” the minister added.

During the meeting, the new US president reportedly criticized Germany for not making adequate contributions to the military alliance, forcing the US to burden larger spending than its fair share.

Under a 2014 agreement, each NATO member should set aside two percent of their GDP for military purposes. However, only the US, Britain, Estonia, Greece and Poland have so far been able to meet the target.

Despite constantly increasing its military spending, Germany has yet to invest the required amount. 

According to the report, Trump’s bill takes 2002 as the starting point for the 2-percent payment and calculates the amount that Germans have fallen short of paying since then plus interest.

American officials argue that in 2002, Merkel’s predecessor, Gerhard Schröder, committed his country to higher military spending.

Sources within the US government have indicated that Trump is preparing similar bills for all NATO members who have not paid their share in full.

“The president has a very unorthodox view on NATO defense spending,” said a source close to Merkel. “The alliance is not a club with a membership fee. The commitments relate to countries’ investment in their defense budgets.”

The Times noted that Merkel “ignored the provocation” but promised to gradually increase spending. She also asked Trump to factor in the money Berlin was spending for international development.

A day after his meeting with Merkel, Trump said in a tweet that 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!"

German Defense Minister Ursula von der Leyen said in response that Trump was wrong to assume that Germany and other NATO members have to spend the entire two percent of their economic output on issues related to the alliance.