WASHINGTON -- Ahead of next week's NATO summit in Brussels, President Trump sent a letter to the prime minister of Norway that made clear he will not relent on his criticism of NATO allies' lagging defense budgets.

Norway, the president wrote, "remains the only NATO ally sharing a border with Russia that lacks a credible plan to spend 2 percent of its gross domestic product on defense." Norway is far from alone as a target of criticism -- the president has reportedly sent similar letters to other countries.

Of the 28 NATO nations, only three besides the U.S. spent the promised 2 percent of GDP on defense last year. Throughout his presidency, Mr. Trump has prodded NATO nations to spend more.

"Many of them have not been even close," he said.

Norway's defense minister said in a statement future spending will increase towards the 2 percent goal. European allies are hoping next week's summit goes better than last month's G7 meeting in Canada, which was marked by tense exchanges between Mr. Trump and other world leaders over trade.

Since then, the trade war has only ratcheted up. Harley-Davidson announced it would move some U.S. motorcycle production overseas to avoid European tariffs and Canada announced retaliatory tariffs on $12 billion worth of U.S. imports.