Luxembourg (AFP) - US President-elect Donald Trump must get up to speed on how Europe works in order to avoid "two years of wasted time" when he assumes his new role, European Commission chief Jean-Claude Juncker said.

"Mr. Trump, during his campaign, said that Belgium was a village somewhere in Europe," Juncker said in his frank remarks to students in Luxembourg on the reality TV star's stunning election victory.

"We must teach the President-elect what Europe is and how it works," Juncker said, adding: "I believe we'll have two years of wasted time while Mr. Trump tours a world he doesn't know."

Juncker said that Trump had called NATO into question, which could have "harmful consequences" because it is the model of Europe's defence.

The populist had also "taken a view of refugees and non-white Americans that does not reflect European convictions and feelings", he added.

In June, Trump caused mirth on the continent when he called Belgium a "beautiful city", apparently confusing the country with its capital Brussels-- the seat of the European Union and NATO.

"The Americans, as a general rule, have no interest in Europe," Juncker said in his remarks to the students.

On Wednesday, after Trump's shock victory over rival Hillary Clinton, Juncker and European Council Chief Donald Tusk invited him to a EU-US summit to discuss issues including terrorism and Ukraine.

"We should spare no effort to ensure that the ties that bind us remain strong and durable," the two said in their letter to the president-elect.