Washington (CNN) President Donald Trump is demanding that Norway ramp up its defense spending as a NATO partner, according to a letter he wrote to Prime Minister Erna Solberg that was obtained by CNN.

The letter was one of a series that Trump sent to NATO allies, including Germany, Belgium, Canada and others, demanding they boost spending and threatening to shift the US military presence in Europe if they do not. The letters were first reported in The New York Times on Monday.

Two defense officials told CNN on Tuesday that Defense Secretary James Mattis has also sent a letter to his British counterpart Gavin Williamson, warning that the UK's position in the world was at risk of erosion unless the it upped its defense spending beyond current levels of about 2% of GDP.

Increasing defense spending beyond the NATO recommended minimum is currently a major topic of debate within the UK government.

The letters fit a pattern of Trump critiques of NATO, particularly of defense spending by other members. NATO members committed to spend 2% of their gross domestic product on national defense at a 2014 summit in Wales, and Trump has repeatedly bemoaned that NATO allies have not fulfilled this commitment.

Collective defense

The transatlantic alliance, founded in 1949, is built on the idea of collective defense - that when one of the 29 members is attacked, all of them will come to its aid. That tenet is embedded in the group's Article 5, which has only been invoked once, after the US was attacked on Sept. 11, 2001.

In advocating for increased defense spending, Trump joins his predecessors Barack Obama and George W. Bush, both of whom also lobbied NATO allies to spend more on defense. Robert Gates, who served as defense Secretary under both Obama and Bush, warned in 2011 that the US could lose interest in NATO if allies did not increase investment in their militaries.

In the letter to Solberg, Trump writes, "Norway is the 'eyes and ears' of the northern flank" of NATO yet "remains the only NATO Ally sharing a border with Russia that lacks a credible plan to spend 2 percent of its domestic product on defense."

"I understand domestic political pressures, as I myself have expended considerable political capital to increase our own defense spending," the letter says, also asking for a "strong recommitment by Norway" to meet agreed-upon goals.

Trump also penned what one senior German official called a "not so friendly" letter to Chancellor Angela Merkel stressing the need for Germany to devote more resources to the defense of Europe.

Political pressure

"Continued German underspending on defense undermines the security of the alliance and provides validation for other allies that also do not plan to meet their spending commitments, because others see you as a role model," the letter said, as read to CNN by a source.

Trump went on to say he understands "domestic political pressures against greater government expenditures as I also expended considerable political capital to increase America's defense spending."

"It will, however, become increasingly difficult to justify to American citizens why some countries do not share NATO's collective security burden while American soldiers continue to sacrifice their lives overseas or come home gravely wounded," the letter noted.

The letter did not come as a shock, but the same German official said that Trump's failure recognize the fact that German soldiers are also participating in overseas missions and have suffered casualties was particularly "bad."

Germany sent troops to Afghanistan as part of the NATO response to the Sept. 11 attacks. With 1,300 soldiers there now, Germany is the second largest contributor to the NATO mission in Afghanistan, where they advise local troops in the northern part of the country.

Trump's insinuation that Germany is failing to live up to its responsibilities as a "role model" was also seen as particularly out of touch, the official said.

'He doesn't seem to care'

"He [Trump] thinks you can just press a button and things are different. He thinks the Chancellor can just tell other European countries what to do," the official said. "Even internally, it is extremely difficult to make such changes. He doesn't seem to care -- he just wants to get that deliverable."

The White House declined to comment on presidential correspondence, but a National Security Council spokesman told CNN, "The President is committed to the alliance, as he has stated repeatedly. The President has also been clear we expect our allies to shoulder their fair share of our common defense burden and to do more in areas that most affect them. There is no better way to signal NATO's resolve than for every ally to allocate the resources necessary to share their burden of our collective defense."

Norwegian Defense Minister Frank Bakke-Jensen defended his country's commitment to meeting defense spending goals in a statement on Tuesday, noting that burden-sharing will be a key issue at the upcoming NATO summit in Brussels.

"Our investment share is currently at 27 percent, well above the NATO target. This has allowed for investments in important capabilities, such as the F-35 combat aircraft, P-8 maritime surveillance aircraft and submarines in order to increase Allied and Norwegian security," the statement said.

"The government has recently decided that a premise for the next long term plan will be to further increase Norway's defense spending towards the 2 percent goal. Burden-sharing will be a key issue at the NATO Summit in July, and Norway looks forward to continued discussions on this issue," it added.

Fever pitch

Tensions between the Trump administration and Europe are reaching a fever pitch ahead this month's NATO summit, on the heels of Trump's announcement of tariffs against European allies and his comments denouncing the transatlantic alliance.

"There is a Trump's hell where NATO is as bad as NAFTA and EU worse than China," a senior European official said. "The transatlantic relationship, which all around the table we consider a given, is not a given."

The official added, "We now have a major crisis."