Nato leaders gather in The Hague on Wednesday for a summit tailor-made for US President Donald Trump, with European allies hoping a pledge to hike defence spending will prompt him to dispel doubts about his commitment to the alliance.

The summit is expected to endorse a higher defence spending goal of five 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 war in Ukraine.

Nato officials are hoping the conflict between Israel and Iran, and the US bombing of Iranian nuclear sites at the weekend, will not overshadow the gathering, hosted by alliance Secretary General Mark 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 defence 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 two 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 defence - such as troops and weapons - and 1.5 percent on broader defence-related measures such as cyber security, protecting pipelines and adapting roads and bridges to handle military vehicles. (Reuters)