With U.S. Secretary of State Rex Tillerson visiting NATO for the first time, Germany pushed back Friday against U.S. President Donald Trump's repeated demands that allies increase their military spending.

Tillerson is the third Trump emissary to visit NATO headquarters in recent weeks and insist that allies show his boss the money by devising specific plans to reach a previously agreed target of spending 2 percent of annual GDP on defense.

U.S. Secretary of Defense James Mattis and Vice President Mike Pence brought the same message on separate visits in February.

But exasperation has been rising in Germany, especially after Trump marked Chancellor Angela Merkel's visit to Washington by tweeting that Berlin “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 officials, including Defense Minister Ursula von der Leyen, rejected that assertion and fired back that Trump did not understand how NATO's finances actually work.

German Foreign Minister Sigmar Gabriel, who like Tillerson was attending his first NATO ministerial gathering, told journalists in Brussels that the U.S. was wrongly interpreting a 2014 declaration by NATO leaders in Wales to step up efforts to reach the 2 percent goal. He said that while Germany was working to increase military spending, the target was not mandatory.

"Germany has to invest more in its defense capabilities ... we have already increased defense spending by 13.7 percent under NATO criteria and will it increase by a further 7.9 percent," Gabriel said. "It’s necessary because our army is not in a responsible shape."

"However," he said, "it is important to correctly quote the Wales declaration. Its guidelines say members should lean towards a 2 percent spending, but it is at no point written that this is a fixed goal and that every member state should invest 2 percent of its GDP in defense."

Germany is the EU's biggest economy and currently spends about €37 billion on its military, roughly 1.2 percent of GDP, according to NATO statistics. To reach the 2 percent target would require nearly doubling that spending — an enormous increase in real terms.

Gabriel noted that Germany would have to increase its spending by nearly the amount of France's total annual military spending, and that it would be hard to know what to do with all of the money.

"The idea that Germany in a few years will spend €70 billion each year on the army is an idea that I consider absurd," he said. "It’s particularly absurd if we look at France which spends €40 billion but has also a nuclear program included in it. I would honestly not even know where to put all the aircraft carriers we would buy with €70 billion."

'Nonsense'

Gabriel remarked that some countries meeting the target could not afford to pay essential social benefits to their citizens — a comment probably aimed at Greece. One of only five NATO countries to meet the target, it has in recent years relied on a bailout largely financed by Germany to keep its economy from collapsing.

"The nonsense of such an assumption is evident if we look at those countries that are spending 2 percent of their GDP on defense but cannot even pay for pensions of their citizens," Gabriel said.

The Wales declaration called on allies not meeting the 2 percent spending target to "halt any decline in defense expenditure; aim to increase defence expenditure in real terms as GDP grows; aim to move towards the 2 percent guideline within a decade with a view to meeting their NATO Capability Targets and filling NATO's capability shortfalls."

Other NATO allies said they had reversed military cuts and were working to raise spending; some echoed Gabriel's point that contributions could not be measured in money alone. "We have already reversed course, making sure that cuts to in defense were stopped," said Italian Foreign Minister Angelino Alfano, adding that other contributions, such as Italian search-and-rescue operations for migrants in the Mediterranean, should also count.

NATO Secretary-General Jens Stoltenberg has worked to win support for NATO from Trump and his aides, stressing that allies are already working hard to both raise spending and to address another demand of the Trump administration — doing more to combat terrorism. He will visit the White House in mid-April to prepare for a NATO summit in late May.

Tillerson echoed Trump's call for NATO to focus more on terrorism, and he issued a statement critical of Russia, which continues to support an ongoing military conflict in eastern Ukraine.

But some diplomats pushed back, arguing that countries like Russia, North Korea and Iran pose a far greater danger than terrorists and should be the real focus of a military alliance like NATO.

"NATO is not a CT (counter-terrorism) operation," one NATO diplomat said, noting that the recent attack on Westminster carried out by a U.K. citizen acting alone was a matter for national police.