United States President Donald Trump has issued strong criticism of NATO after arriving in Ankara for the Alliance summit, while expressing optimism that the war in Ukraine could end soon.

During a statement to reporters, Trump said he was "very disappointed" with NATO, claiming that the allies did not support the United States after the actions taken against Iran.

"We'll see. I was very disappointed with NATO. Frankly, if the summit hadn't been held in Turkey, where my friend is a very strong leader, I might not have attended," Trump declared, praising the organization of the summit in Ankara and the relationship with Turkish President Recep Tayyip Erdogan.

The US president stressed that the US has invested "trillions of dollars" in the defense of NATO member countries, but added that it did not receive the support it expected from allies.

"We have invested trillions of dollars to protect Europe, Canada and other countries. You would expect them to be willing to help us, but in fact they were not," he said.

Trump also commented on the stance of former British Prime Minister Keir Starmer, saying that his statement that the United Kingdom would only help after the war was over was "very unpopular."

"We didn't need anyone's help. In a way, I was testing people to see if they would be there or not," the US president added.

Asked about the war in Ukraine, Trump announced that he had had a "very good conversation" with Russian President Vladimir Putin, and expressed his belief that the conflict could end soon.

"I think we will solve it. I hope a solution will be reached very soon," he said.

Trump is expected to meet with Ukrainian President Volodymyr Zelenskyy on Wednesday on the sidelines of the NATO summit in Ankara.