"The military will as of tomorrow raise its preparedness in Norway," Prime Minister Jonas Gahr Store told reporters.

"We currently have no reason to believe that Russia wants to involve Norway or any other country directly in the war," he said.

"But the war in Ukraine means it is necessary for all NATO countries to be increasingly on their guard."

Norway has overtaken Russia as Europe's main natural gas supplier following Moscow's invasion of Ukraine.

The Scandinavian country has already boosted security around strategic sites after mysterious drone flights were observed near its offshore oil and gas platforms and the suspected sabotage of Nord Stream's Baltic Sea pipelines.

In recent weeks, several Russians have been arrested in Norway for illegally flying drones or violating photo restrictions in restricted areas.

Last week, Norway's counter-intelligence service also announced the arrest of a man suspected of being a Russian undercover agent, who experts said could be a senior officer from the GRU military intelligence service.

"I must stress that nothing has happened in the last day or days that has prompted us to raise our preparedness now. It is an evolution over time which has led us to take this step," Store said.

Defence Minister Bjorn Arild Gram said the move would include measures related to logistics, communications security and security at military installations.

Norwegian defence chief General Eirik Kristoffersen said the armed forces would redefine their priorities to address the changing security situation.

One or more F-35 stealth fighter jets that were due to be stationed for development tests in the United States will now remain in Norway, he said.

The government will also push the United States to accelerate the delivery of state-of-the-art P-8 maritime patrol aircraft, Kristoffersen added.

Amid rising tensions between Russia and the West, Oslo has already increased the military budget and intelligence efforts in Northern Norway, where it shares a 198-kilometre (123-mile) border with Russia.