North Korea lashed out Friday at the arrival of a US aircraft carrier battle group in South Korea, calling it a provocation and again raising the specter of using nuclear weapons to defend itself. Emboldened by its advancing nuclear arsenal, North Korea has increasingly issued threats to use such weapons preemptively. 

But the North is still outgunned by US and South Korean forces, and experts say it is unlikely to use its nukes first, though it will continue to upgrade those arms without returning to diplomacy for the time being.