The former Norwegian prime minister arrived in the post in 2014 and had been handed a previous two-year extension to 2020.

In a statement, the alliance said its 29 members had "expressed their support for the Secretary General's work to adapt and modernise NATO and ensure it remains fit for the future."

Stoltenberg, 60, vowed to pursue that goal and tweeted: "Honoured by the decision by the #NATO Allies to extend my term as Secretary General until 30 September 2022."

The 70-year-old organisation faces numerous challenges, not least demands from US President Donald Trump that Washington's European allies increase their share of defence spending.

NATO has also deplored Russia's threat to deploy new intermediate range missiles which can reach reach European capitals following the collapse of the 1987 Intermediate-Range Nuclear Forces (INF) treaty.

On a March 1 trip to Bulgaria Stoltenberg appealed to Russia to step back from its threat and return to compliance with the treaty.

He warned that the world needed to be "prepared for a world without the INF treaty and with more Russian missiles."

Days later, Russian President Vladimir Putin officially suspended Moscow's participation in the treaty.

Stoltenberg's tenure at NATO has also coincided with rising tensions with Russia following Moscow's March 2014 annexation of Crimea and the conflict in eastern Ukraine which has left some 13,000 dead over five years.

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