Long-serving NATO leader Jens Stoltenberg has no plans to extend his mandate for a fourth time and expects to be replaced in October, the military alliance said Sunday.

"The mandate of Secretary General Jens Stoltenberg has been extended three times and he has served for a total of almost nine years," spokeswoman Oana Lungescu said.

"The Secretary General's term comes to an end in October of this year and he has no intention to seek another extension of his mandate."

The 63-year-old former Norwegian prime minister took office at NATO's Brussels headquarters on October 1, 2014 and has overseen the Western alliance through several international crises.

Under his term in office the last NATO personnel and US forces left Afghanistan in August 2021, shortly before the capital Kabul fell to victorious Taliban forces, who revived their Islamist regime.

Stoltenberg has also overseen NATO's response to Russia's February 2022 invasion of Ukraine and the ongoing war, the most brutal on European soil since the 1940s.

He has been a respected secretary general and in particular a bridge between the European allies and Washington under former US president Donald Trump, a frequent NATO critic.

Diplomats from several NATO allies had speculated that Stoltenberg's mandate could be extended once again until a summit next year to mark 75 years of the alliance.

But, after a visit to Washington last week, his office announced he would not seek another extension.