U.S. stock index futures were slightly lower on Tuesday as investors stayed away from risky assets after China deployed retaliatory tariffs against the world's biggest economy. Minutes after President Donald Trump's 10% tariff on China kicked in at 12:01 a.

The stock market is waiting anxiously for the next move on tariffs as China’s retaliation to levies from the U.S. has put markets on edge amid fears of an escalating tit-for-tat trade war between the two powers.

China's finance ministry said it would impose levies of 15% for U.S. coal and 10% for crude oil, farm equipment and some cars.