European Commission chief Jean-Claude Juncker said less is more when it comes to the delicate trans-Atlantic trade truce and U.S. President Donald Trump’s social-media habits.

Amid signs of fresh EU-U.S. commercial tensions, Juncker said Trump’s penchant for using Twitter to make surprise announcements risks putting a July 2018 cease-fire—halting threatened tariffs against European cars—a key stroke away from being broken.

“I hope the provisional end of the trade war between Europe and the U.S. will also withstand the Twitter energy of the White House,” he told a conference in Brussels. “That is important.”

The quip by Juncker, president of the European Union’s executive arm, comes at a sensitive time for trans-Atlantic ties.

Envoys from EU member states on Thursday will discuss giving the commission the go-ahead for talks with the U.S. to slash industrial tariffs. The negotiating mandate is a bid to show Trump progress in enacting the political accord he reached with Juncker nine months ago.

Separately, the Trump administration this week announced a plan to impose tariffs on $11 billion of European goods as a result of alleged subsidies to plane maker Airbus SE. The commission reacted by saying it was preparing similar action against American products in a tit-for-tat case at the World Trade Organization concerning aid to Boeing Co.