Gammera The Invincible (1966)

Starring Albert Dekker, Brian Donlevy, Diane Findlay, John Baragrey, Dick O'Neill
Directed by Noriaki Yuasa
(actor & director credits courtesy IMDB.com)

An atomic blast in the Arctic unleashes a giant prehistoric turtle that devastates all in its way in search of energy to feed on, leading the world's nations to convene to find a way to stop the creature.

The first in a long series of Gammera movies from Japan, and to the best of my knowledge, the only installment in the series in which Gammera is a villain (it battled other monsters menacing the universe in the sequels).  This isn't the original version of the film but the Americanized re-release, with Dekker and Donlevy playing members of the U.S. military brass trying to lend their support to the Japanese authorities, but the emphasis is still on a young Japanese boy's friendship with the monster turtle, paving the way for a series of young co-stars in Gammera movies to come.  It's a fun and entertaining concoction, with a much lighter tone than other Japanese monster epics, featuring some more than serviceable special effects that hold up fairly well.