By Phil Stewart and Pavel Polityuk RAMSTEIN AIR BASE, Germany/KYIV (Reuters) -The United States and its allies promised to send more heavy weaponry to Ukraine during talks at a German air base on Tuesday, brushing off a threat from Moscow that their support for Kyiv could lead to nuclear war. But in a move that sharply raises the economic stakes for the West, Russia said it would turn off gas supplies to Poland on Wednesday morning after weeks of threats that it would cut off supply to countries who refuse to pay for gas in roubles. Poland, a key NATO ally of the United States and among the Eu…