America's National Security Agency (NSA) bugged European Union offices in Washington and infiltrated the EU's computer networks, German magazine Der Spiegel reported on Saturday.
According to Spiegel, a "top secret" document from 2010, obtained by former NSA contractor Edward Snowden,shows the agency installed bugs in the building housing the EU representative's office in downtown Washington, DC.
It also infiltrated the office's computer network, allowing it to "access discussions in EU rooms as well as emails and internal documents on computers."
The revelations could further strain relations between Europe and the U.S. already hurt by previous disclosures about the extent of NSA spying activities outside the United States.
(Read More: Snowden's Options to Evade US Arrest Narrowing)
On Saturday, Martin Schulz, the president of the European Parliament demanded a speedy clarification from Washington.
"I am deeply worried and shocked about the allegations of U.S. authorities spying on EU offices. If the allegations prove to be true, it would be an extremely serious matter which will have a severe impact on EU-U.S. relations," he said.
According to the Associated Press, Germany's Justice Minister Sabine Leutheusser-Schnarrenberger compared the alleged bugging to "methods used by enemies during the Cold War."
Meanwhile, Luxembourg Foreign Minister Jean Asselborn told Spiegel Online, the allegations were a breach of trust and that the EU and its diplomats are not terrorists.
"If these reports are true, it is abominable," he said.
The Spiegel report published on Saturday said the NSA also spied on the EU representative to the United Nations in New York and on offices in Brussels.