US President Donald Trump said any decision on reducing American troop deployments in Europe would depend on future developments, keeping NATO allies uncertain about Washington's long-term military presence ahead of the alliance's summit in Ankara.
Speaking before departing for the summit, Trump said the United States had invested heavily in NATO and that European allies should contribute more to the alliance's defence efforts.
"We've invested in NATO. Allies should be willing to help," Trump said, according to the White House Rapid Response account.
Trump also said the US had used its actions during the Iran conflict to assess the commitment of its allies.
"I was testing allies when we went into Iran," he said, adding that countries including France and Germany had declined to participate.
When asked whether his administration was considering reducing the number of US troops stationed across Europe, Trump did not give a direct answer.
"We'll have to see," he said.
NATO leaders are set to meet in Ankara for discussions on defence spending, military cooperation and continued support for Ukraine.
The summit is also expected to feature announcements of arms deals worth tens of billions of dollars as member states seek to demonstrate stronger defence commitments amid pressure from Washington and continuing security concerns linked to Russia.
Trump's remarks come as disagreements persist between the United States and several NATO allies over defence burden-sharing and America's long-term military role in Europe. His latest comments are likely to add to uncertainty among alliance members as high-level talks get underway.