The largest Nato exercise planned for this year, Cold Response 2022 will bring together around 30,000 troops from 27 nations.

From mid-March to early April, they will train on land, at sea and in the air, deploying troops to help an attacked Nato country.

As usual, member states of the Organisation for Security and Cooperation in Europe (OSCE), including Russia, have been invited to send observers to follow the exercise, which was planned long before Russia's military offensive in Ukraine last week.

"Russia thanked for the offer to send observers during Cold Response 22 but declined," Norwegian army spokesman Preben Aursand told AFP.

The reasons for the refusal were not known.

Foreign troops have already arrived in Norway, going back several weeks, even months, and more are expected in the coming days.

The number of troops announced to participate has fluctuated: initially it was put at more than 40,000, it is now estimated at around 30,000.

"Some nations have chosen to use their troops in other ways," said Preben Aursand, without giving further details.

In addition to Nato members, Finland and Sweden, two nations that are officially non-aligned but where the debate over membership has been reignited by the Russian invasion of Ukraine, are both expected to send contingents.

The US and the UK will deploy one carrier battle group each during the exercise.