Russia vowed on Thursday to retaliate for a plan by Norway to more than double the number of U.S. Marines stationed there.
Oslo announced on Tuesday that it would ask the United States, its NATO ally, to send 700 Marines to train in Norway from 2019, against 330 at present, and said the additional troops would be based closer to the Russian border.
"This makes Norway less predictable and could cause growing tensions, triggering an arms race and destabilizing the situation in northern Europe," the Russian Embassy said in a statement on its Facebook page.
"We see it as clearly unfriendly, and it will not remain free of consequence."
Oslo has grown increasingly concerned about Russia since Moscow annexed Crimea from Ukraine in 2014. However, it has added that it does not regard its much larger neighbor as a direct threat.
The U.S. Marines were scheduled to leave at the end of this year after an initial contingent arrived in January 2017 to train for winter conditions. They are the first foreign troops to be stationed in Norway since World War Two.
The initial decision to welcome the Marines had prompted Moscow to say it would worsen bilateral relations and escalate tensions on NATO's northern flank.
On Wednesday, Russia's Northern Fleet launched a large naval exercise in the Arctic Barents Sea. Later this year, Norway will host its biggest NATO maneuver in decades.
