By Jeff Mason and Sabine Siebold

THE HAGUE (Reuters) -President Donald Trump cast doubt on Tuesday over the United States' commitment to defending its NATO partners, suggesting there were "numerous" definitions to the cornerstone of the alliance's mutual defence pact.

Trump was speaking to reporters en route to a NATO summit in the Netherlands, a two-day gathering which is intended to signal to Russian President Vladimir Putin that NATO is united, despite the U.S. president's past criticism, and determined to expand and upgrade its defences to deter any attack from Moscow.

However, pressed by reporters on Air Force One over whether he remained committed to mutual defence among allies as set out by NATO's Article 5, Trump responded:

"I'm committed to saving lives. I'm committed to life and safety. And I'm going to give you an exact definition when I get there."

NATO Secretary General Mark Rutte played down the comments. "I have no doubt that the U.S. is totally committed to NATO, totally committed to Article 5," he told reporters at the summit venue in The Hague.

NATO'S RUTTE HAILS TRUMP'S 'DECISIVE ACTION IN IRAN'

Trump also posted a screenshot of a message from Rutte congratulating him on his "decisive action in Iran" and getting all NATO allies to agree to spend more on defence.

"Europe is going to pay in a BIG way as they should, and it will be your win," Rutte's message read, indicating the effort he has put into keeping on the right side of Trump and ensuring the summit is a success.

The summit and its final statement will be focused on heeding Trump's call to spend 5% of GDP on defence - a significant jump from the current 2% goal. It is to be achieved both by spending more on military items and by including broader security-related spending in the new target.

Trump singled out Spain for criticism after Spanish Prime Minister Pedro Sanchez declared Madrid did not need to meet the new spending target.

TRUMP SET TO MEET ZELENSKIY

Trump is expected to meet Ukraine's President Volodymyr Zelenskiy for talks during the summit. Zelenskiy has said he wants to discuss substantial purchases of weaponry including Patriot missile defence systems as well as sanctions and other ways to put pressure on Putin.

Zelenskiy warned European NATO members on Tuesday that they risked being attacked by Russia if it was not defeated in Ukraine.

"Russia is even planning new military operations on NATO territory – meaning your countries," Zelenskiy told a defence industry event on the sidelines of the summit, hours after Russian missiles killed at least 17 people in southeast Ukraine.

Zelenskiy said it was essential that Ukraine lead in drone technology, which has shaped the battlefield and developed at breathtaking pace in the 40 months the war has lasted so far.

RUSSIA CRITICISES NATO'S SPENDING BOOST

The Kremlin accused NATO of being on a path of rampant militarisation and portraying Russia as a "fiend of hell" in order to justify its big increase in defence spending.

Russia has cited its neighbour's desire to join the U.S.-led transatlantic defence pact as one of the reasons why it invaded Ukraine in 2022.

NATO was founded by 12 Western countries in 1949 to resist the threat from the communist Soviet Union.

Russia denies any plan to attack the alliance, which now boasts 32 members, but Kremlin spokesman Dmitry Peskov said it was "largely a wasted effort" to assure the grouping of this because it was determined to demonise Russia.

"It is an alliance created for confrontation ... It is not an instrument of peace and stability," he said.

(Additional reporting by Lili Bayer, Bart Meijer and John Irish in The Hague; Writing by Keith Weir; Editing by Kevin Liffey and Toby Chopra)