(Reuters) -Donald Trump's plan to impose new 25% tariffs on steel and aluminium boosted share prices of U.S. producers in pre-market trading, while shares of producers abroad fell, with traders citing worries of disrupted flows and a reduction in demand. The tariffs, which Trump said would be announced on Monday and be in addition to existing duties, sparked warnings from Asia and Europe about the impact on prices, profitability and volumes and broader worries that they could push up inflation and drag on economic activity. Canada, Brazil, Mexico, South Korea and Vietnam are the biggest sellers of steel into the U.S., according to American Iron and Steel Institute data, while Canada is the dominant supplier of imported aluminium.