(Reuters) -European shares opened lower on Monday, after rounding off monthly gains in May, as U.S. President Donald Trump's new tariff plans threatened to rekindle global trade tensions.

The continent-wide STOXX 600 was down 0.2% as of 0708 GMT.

Late on Friday, Trump said he planned to increase tariffs on imported steel and aluminum to 50% from 25%, to which the European Union said it was prepared to retaliate.

Steel companies in Europe fell, with ArcelorMittal down 1% and conglomerate Thyssenkrupp down 1.1%.

The tariffs, which can impact automobiles, weighed on stocks of carmakers, with the sector down 1.2%.

Meanwhile, Sanofi agreed to buy U.S.-based Blueprint Medicines Corporation, paying $129 per share, representing an equity value of approximately $9.1 billion. Shares in the French pharma group were slightly lower.

This week, the spotlight will be on the European Central Bank, which will announce its interest rate decision on Thursday.

Comments from Federal Reserve Chair Jerome Powell and ECB President Christine Lagarde will be on tap, alongside a slew of economic data out of the trade bloc.

(Reporting by Purvi Agarwal in Bengaluru; Editing by Mrigank Dhaniwala)