From the April 18th Economist, in the article “Putin’s targeted strike: The meaning of Russia’s weapons sale to Iran”:
In July 2013 Russia remained silent when an Israeli air strike destroyed anti-ship cruise missiles that it had recently supplied to Syria and were on their way to Hizbullah. And Israel kept shtum last October when Syrian rebels released footage of the involvement of Russian intelligence officers at a Syrian military listening post on the Golan Heights that had been overrun.
Israel kept shtum. With the adjective shtum ‘silent, mute’ — an item that, apparently, few Americans know, unless they have some experience of British English. (The Economist is a British publication.) On the British side, the item is ordinary slang, commonly used in the collocation keep shtum (and in some other contexts). It seems to be derived from Yiddish, though I believe that very few British speakers appreciate that; for them, it’s just slang. So there’s something of a puzzle as to how it became naturalized in BrE but not AmE.