On Monday he'd tweeted the situation was "not fair, nor is it acceptable," and insisted that NATO benefits Europe "far more than it does the U.S."
He added: "NATO countries must pay MORE, the United States must pay LESS. Very Unfair!"
Trump, who has compared the sentiment that underpinned the Brexit vote to leave the EU to his own election, will be making his maiden presidential trip to Britain at a fraught time for May. Foreign Secretary Boris Johnson and Brexit Secretary David Davis resigned within hours of each other in protest of her plan. Trump said might meet with Johnson in the UK despite his resignation.
Trump's visit is expected to attract large protests in London and elsewhere in Britain.
Trump's weeklong trip to Europe will continue with a stop in Scotland before ending with a sit-down in Helsinki with Putin.
He said that of the high-stakes meetings of his trip, "Putin may be the easiest of them all."
"I think that getting along with Russia, getting along with China, getting along with others is a good thing, not a bad thing," he added.
The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied the allegations of election meddling, in spite of evidence to the contrary.
Colvin reported from Washington. AP writer Zeke Miller contributed from Washington.