The letters to NATO allies, including Belgium, Canada, Germany and Norway, have been sent as early as last month, The New York Times newspaper reported on Monday.
"As we discussed during your visit in April, there is growing frustration in the United States that some allies have not stepped up as promised…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 military spending commitments, because others see you as a role model," Trump's letter to German Chancellor Angela Merkel, in particular, said as quoted by the media outlet.
Kremlin spokesman Dmitry Peskov said amid the report that NATO was created in the era of confrontation and was therefore fully tailored to this purpose.
"NATO is the child of the era of confrontation, this is the organization which was created for the purposes of confrontation, it was completely tailored to these purposes… We see that the NATO military infrastructure is approaching our borders. Our position in respect to this process is well-known, it has not changed," Peskov told reporters.
In 2014, NATO allies pledged to end cuts to their defense spending, and gradually increase it toward the alliance's designated 2-percent GDP target within a decade.
Trump has repeatedly called on NATO allies to respect this commitment, threatening, otherwise, to decrease Washington's participation in the programs aimed at ensuring the security of NATO member states.
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So far, only five countries have reportedly met Trump's requirement; Apart from the United States, the United Kingdom, Greece, Poland, and Estonia have confirmed.