Stock market: Dax opens in the red, Trump's tariff threats unsettle markets

Meanwhile, in the oil market, investors' interest is increasingly focused on a possible change in the US government's Russia policy, which could result in new sanctions against the important oil-producing country.

According to a US media report, Trump plans to make a statement on his Russia policy later today. Influential Republican US Senator Lindsey Graham (70) has already hinted at a possible policy shift. "A turning point regarding Russia's invasion of Ukraine is imminent," he told US television network CBS on Sunday.

There was speculation on the market that Trump could announce tougher sanctions against Russia during the course of Monday, including levies on major customers who buy Russian oil.