Stoltenberg's successor when he steps down in October will take office at a crucial juncture, tasked with sustaining NATO members' support for Ukraine's costly defence against Russia's invasion while guarding against any escalation that would draw the alliance directly into a war with Moscow.

"President (Joe) Biden strongly endorses PM Rutte's candidacy to be the next Secretary General of NATO," a U.S. official said.

"PM Rutte has a deep understanding of the importance of the Alliance, is a natural leader and communicator, and his leadership would serve the Alliance well at this critical time."

Depending on the outcome of November's U.S. presidential election, the next NATO boss may have to deal with a second term for Donald Trump, who drew fierce criticism from Western officials earlier this month for calling into question his commitment to defending NATO allies if re-elected.

Founded in 1949 to counter the Soviet Union during the Cold War, NATO is a political and military alliance of countries from North America and Europe.

Enshrined in Article 5 of its founding treaty is the principle of collective defence - the idea that an attack on one member is considered an attack on all.

The Netherlands' longest-serving leader, 57-year-old Rutte has had good relationships with various British, European Union and U.S. leaders - including Trump - during his tenure.

At the weekend, Rutte urged European leaders to "stop moaning and whining and nagging" about Trump and focus instead on what they could do to bolster defence and help Ukraine.

Backing Rutte, the British Foreign Office said he was a well-respected figure across NATO with serious defence and security credentials, and someone who would ensure it remained strong and prepared for any need to defend itself.

A senior French official said Paris also backed Rutte, adding that President Emmanuel Macron had been an early supporter of putting the Dutchman in the role, having sounded him out about the post last year.

APPOINTED BY CONSENSUS

German Chancellor Olaf Scholz also supports Rutte, a government spokesman said.

Having the public support of Washington, the alliance's predominant power, and the three big European nations puts Rutte in a commanding position.

Three diplomats said he has the backing of about 20 NATO members so far. But another senior diplomat cautioned a deal had not yet been reached and another candidate could still emerge.

NATO leaders are appointed by consensus, meaning all 31 members must be on board to reach a final decision.

Poland - a growing military power in Europe - has no position yet, a foreign ministry spokesperson said.

Diplomats see Hungary and Turkey as possible holdouts but there was no immediate comment from them on their positions.

Under Rutte's leadership, Dutch defence spending was cut during years of fiscal austerity. Since Russia's invasion of Ukraine, however, the Netherlands has increased spending, taking them to around 2% of GDP in 2024. Rutte has long been a fierce critic of Russian President Vladimir Putin.

Rutte unexpectedly announced his departure from Dutch politics in July, but remains in post as a caretaker leader while coalition negotiations continue following a Nov. 22 election.

Stoltenberg, a former Norwegian prime minister, has served as NATO chief since 2014. Sweden is set to become NATO's 32nd member.

Estonian Prime Minister Kaja Kallas and Latvian Foreign Minister Krisjanis Karins have also signalled interest in NATO's top job but have not been presented formally as candidates, diplomats say.

Danish Prime Minister Mette Frederiksen, whom some had mentioned as a possible successor, ruled it out on Thursday.