ukraine never, never, ever becomes member of nato. >> reporter: privately, diplomats admit there is little chance of that, anyway. but u.s. officials insist formally ruling out nato membership under russian pressure will not happen. >> however, in pushing back on security proposals that are simply nonstarters for the united states. we will not allow anyone to slam closed nato's open-door policy, which has always been central to the nato alliance. >> reporter: there have also been clear warnings from the u.s. of tough new sanctions if russia launches another invasion of ukraine. u.s. officials say the unspecified sanctions would go much further than previous ones imposed on russia after its annexation of crimea from ukraine in 2014. and would have an overwhelming impact on the russian economy.