The Northern Sea Route has been contested for decades:

https://warontherocks.com/2023/10/getting-sporty-in-russias-...

The US is also upset that Russia had been making its own Arctic ECS claims through proper legal channels:

> Russia has cleared the scientific burden of proof required to have its extended continental shelf claim legitimized. Decades of scientific research, Arctic missions, and information exchanges with Denmark and Canada have resulted in the U.N. Commission on the Limits of the Continental Shelf dubbing the majority of Moscow’s claim to an extended Arctic continental shelf to be “valid.”

https://warontherocks.com/2023/05/russias-gains-in-the-great...