Here's Pentagon press conference about stuff.
First reply (starts at ~00:1:10) addresses this ICBM stuff.
She says it's an experimental rocket, based on the RS-26 (they can't/won't give more details), US was notified beforehand by the Kremlin.
> nuclear posture
> new type of lethal capability [...] certainly a concern to us
Butifel words. Ridiculous gibberish.
There are other interesting things, like anti-personnal mines and such.