If what you are hearing is
[ˌɪnɚˈnæʃənɫ̩], where /nt/ becomes [n], then that one is called assimilation, and it is by no means restricted to the United States alone. Specifically, this is a case of progressive assimilation (left-to-right), in which a later sound becomes more like an earlier one.
It is possible that you should also take into account that a /t/ may often be expressed as a glottal [ʔ] in the syllable coda, and so may appear to you to have been altogether deleted here — especially if the stop is incomplete, as in rapid speech it may well be.
In fact, it is not uncommon to hear “international” pronounced as
[ˌɪ̃ʔɚˈnæʃənɫ̩]. Here the /n/ nasalizes the preceding vowel in regressive assimilation (right-to-left), but is itself deleted.
This might even be a case of reciprocal assimilation where the bleed-over goes both ways.