"I've always had a very good relationship with Vladimir Putin of Russia, but something has happened to him," said President Trump.

Trump has said he wants to use more of a carrot than stick when it comes to convincing Moscow to agree to a ceasefire, but, aside from direct Ukraine-Russia talks and further prisoner of war exchanges, there has been little to no progress on bringing a pause in fighting closer, despite the US president's growing impatience.

But two things could leave the alliance’s secretary general, Mark Rutte, presiding over a more difficult event: President Trump, who is expected to attend, is at odds with allies over Ukraine’s future. And Ukraine itself wants to join NATO, an outcome Mr. Trump opposes.