Taking place in the shadow of Russia's war in Ukraine, the Madrid gathering comes at a pivotal moment for the transatlantic bond after failures in Afghanistan and internal discord during the era of former US president Donald Trump, who threatened to pull Washington out of the nuclear alliance.

Negotiations among an often-fractious organisation are still under way, diplomats said, but leaders also hope to agree to provide more military aid to Ukraine, increase joint defence spending, cement a new resolve to tackle China's military rise and put more troops on stand-by to defend the Baltics.

Spain is also pushing for more NATO focus on the southern flank to address migration and militant groups in the Sahel region of Africa.

The leaders of Australia, New Zealand, Japan and South Korea are expected to attend part of the summit, part of a broader US strategy for a more assertive Western presence in the Indo-Pacific region to counter China.

"We will do more to ensure we can defend every inch of allied territory, at all times and against any threat," NATO Secretary-General Jens Stoltenberg said last week.

Although British and US officials have advised against a Baltic request for permanent multinational forces in the region, the summit is likely to settle on a compromise of promising rapid reinforcements.

Germany has already said it will put more troops at the ready to defend Lithuania should Russia seek to seize NATO territory and Britain is expected to do the same for Estonia, while Latvia is looking to Canada to pledge more troops there.

NATO - created in 1949 to counter the Soviet threat - is under no treaty obligation to defend Ukraine, as the former Soviet republic is not a NATO member.

But Russian President Vladimir Putin's February 24 invasion has sparked a geopolitical shift as once neutral countries Finland and Sweden seek to join NATO and Ukraine has formally become a candidate to join the European Union.

If accepted, Finland and Sweden's inclusion into NATO would bring about the expansion of the alliance that Putin aimed to prevent.

"I think it sends an important message to Putin. And I think it would actually significantly strengthen the alliance," US Senator Angus King said of Finland and Sweden, following a trip to Finland, Latvia and Turkey.

However, Turkey is also testing that unity, angered by what it says is Helsinki and Stockholm's support for Kurdish militants and arms embargoes on Ankara.

A Turkish government official involved in the talks between the three countries and NATO's Stoltenberg told Reuters it would be difficult to reach a deal at the summit, saying that Sweden and Finland must first address Turkish concerns.

"There were meetings, but unfortunately steps we expected are not being taken," the official said.

Sweden has set up a process for ongoing consultations, diplomats said. But two senior NATO diplomats said the dispute was less about technical benchmarks and more about politics.

Australian Associated Press