Speaking to reporters en route to the summit in The Hague, Trump said there were "numerous" definitions of Article 5 of the Washington Treaty, which declares that an attack against one NATO member shall be considered an attack against all of them.
"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," Trump said.
"I have no doubt that the US is totally committed to NATO, totally committed to Article 5," Rutte told reporters.
The two-day summit in the Netherlands is intended to signal to Russian President Vladimir Putin that NATO is united and determined to expand and upgrade its defences to deter any attack from Russia.
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 five per cent of GDP on defence - a significant jump from the current two per cent goal.Â
It is to be achieved both by spending more on military items and by including broader security-related spending in the new target.
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 military spending.
Russia has cited its neighbour's desire to join the US-led transatlantic military alliance as one of the reasons why it invaded Ukraine in 2022.
NATO was founded by 12 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.
