I saw part of this as a kid (many years ago) and I can't remember much of it. However it was sixties high tech adventure movie, with lots of henchmen running about with submachine guns in glorious technicolor overalls. As far as I can make out it had to do with the Chinese building tunnels (think railway tunnels) under the USA in onder to blow everything to bits with nuclear bombs.
I haven't been able to track it down and it bugs me ever since!
That would be Battle Beneath the Earth (http://www.imdb.com/title/tt0061387/) aka "Battle Beneath the Sea" from 1967.
That definitely seems to be it! thanks