Very unlikely that China, Russia, or US (and its allied countries) would misidentify a nuclear attack. There are massive surveillance resources dedicated toward

Very unlikely that China, Russia, or US (and its allied countries) would misidentify a nuclear attack. There are massive surveillance resources dedicated toward atmospheric sampling, etc. etc. and a non-nuclear explosion would be correctly identified as such in very short order. On top of that, even a 30-meter asteroid impact would release more energy on impact than a typical nuclear warhead. A 250-meter asteroid might release more energy than ALL nuclear weapons used in the world to date.