The summit is expected to endorse a higher defense spending goal of 5 percent of GDP - a response to a demand by Trump and to Europeans' fears that Russia poses an increasingly direct threat to their security following the 2022 invasion of Ukraine.

NATO Secretary General Mark Rutte acknowledged that it was not easy for countries to find the money for extra defense spending but said it was vital to do so.

"There is absolute conviction with my colleagues at the table that given this threat from the Russians, given the international security situation, there is no alternative," he told reporters on Wednesday morning.

NATO officials are hoping the conflict between Israel and Iran, and the U.S. bombing of Iranian nuclear sites at the weekend, will not overshadow the gathering, hosted by Rutte in his home city.

Trump has threatened not to protect NATO members if they fail to meet spending targets and he raised doubts about his commitment again on his way to the summit by avoiding directly endorsing the alliance's Article 5 mutual defense clause.

Speaking to reporters aboard Air Force One, he said there were "numerous definitions" of the clause. "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," he said.

The new target - to be achieved over the next 10 years - is a big increase on the current goal of 2 percent of GDP, although it will be measured differently. It would amount to hundreds of billions of dollars in extra annual spending.

Countries would spend 3.5 percent of GDP on core defense - such as troops and weapons - and 1.5 percent on broader defense-related measures such as cyber security, protecting pipelines and adapting roads and bridges to handle military vehicles.

All NATO members have backed a statement enshrining the target, although Spain declared it does not need to meet the goal. Madrid says it can meet its military commitments to NATO by spending much less - a view disputed by Rutte.

But Rutte accepted a diplomatic fudge with Spanish Prime Minister Pedro Sanchez as part of his intense efforts to give Trump a diplomatic victory and make the summit go smoothly.

Trump gave an unusual insight into those efforts on Tuesday by posting a private message in which Rutte lavished praise on him and congratulated him on "decisive action in Iran".

"You will achieve something NO American president in decades could get done," Rutte told Trump.

"Europe is going to pay in a BIG way as they should, and it will be your win."

To satisfy Trump, Rutte has also kept the summit and its final statement short and focused on the spending pledge.

The text is expected to cite Russia as a threat and reaffirm allies' support for Ukraine but not dwell on those issues, given Trump has taken a more conciliatory stance towards Moscow and been less supportive of Kyiv than his predecessor, Joe Biden.

Ukrainian President Volodymyr Zelenskiy had to settle for a seat at the pre-summit dinner on Tuesday evening rather than a seat at the main meeting on Wednesday, although Trump said he would probably meet with Zelenskiy separately.

Zelenskiy and his aides have said they want to talk to Trump about buying U.S. weapons including Patriot missile defense systems and increasing pressure on Moscow through tougher sanctions.

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