By Ben Deighton and Annika Breidthardt

BRUSSELS/BERLIN, June 30 (Reuters) - The European Union has demanded that the United States explain a report in a German magazine that Washington is spying on the group, using unusually strong language to confront its closest trading partner over its alleged surveillance activities.

A spokeswoman for the European Commission said on Sunday the EU contacted U.S. authorities in Washington and Brussels about a report in Der Spiegel magazine that the U.S. secret service had tapped EU offices in Washington and Brussels and at the United Nations.

"We have immediately been in contact with the U.S. authorities in Washington D.C. and in Brussels and have confronted them with the press reports," the spokeswoman said.