The president said at a press conference with British Prime Minister Theresa May that new economic restrictions would begin to bite, possibly even during negotiations between the two sides.
Europe's traditional centrist coalition lost its majority, with far-right populist parties and liberal, pro-European Union parties gaining ground. The results suggest a complicated future for the EU.
"I believe this is the first time since the beginning of European integration that this European Union could actually break apart," center-left Dutch politician Frans Timmermans warned this month.
The worldview that nations should promote their interests over those of the international community poses a challenge for Germany, a country built on partnerships.
As many pro-Brexit voters express nostalgia for an earlier time in the country's history, the governing Conservative Party could face a humiliating defeat in European elections.
After Britain voted in 2016 to leave the European Union, many feared other countries would follow and the EU might collapse. But Brexit's chaotic example has helped strengthen the EU's standing.
The Trump administration is preparing tariffs on billions of dollars in imports from Europe in retaliation for subsidies of Airbus jets. The levies would hit products ranging from aircraft to wine.