"NATO wasn't there when we needed them, and they won't be there when we need them again. Remember Greenland, the big, poorly managed piece of ice," Trump writes on Truth Social.
Trump said before the meeting that NATO had been "tested and failed" in connection with the US and Israel's war against Iran.
NATO Secretary General Jens Stoltenberg now says that "some" members have failed.
"Some of them, yes, but a large majority of European countries, and that's what we discussed today, have done what they previously promised in a case like this," he told CNN, according to Reuters.
Stoltenberg added that his conversation with Trump had been open and honest.
Ahead of the meeting, White House press secretary Karoline Leavitt said that Trump intended to talk about a possible American withdrawal from NATO.
The president is dissatisfied that NATO countries have not wanted to help open the Strait of Hormuz, which he says has been closed by Iran as a result of the US and Israel's war against Iran.
Trump is also considering punishing specific NATO countries that he believes were unhelpful during the war, sources within his administration told The Wall Street Journal, by moving American troops from those countries to others deemed more helpful.
