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.
According to Spiegel, one operation may have even targeted the EU Council of Ministers.
"A little over five years ago, EU security experts noticed several telephone calls that were apparently targeting the remote maintenance system in the Justus Lipsius Building where the EU Council of Ministers and the European Council is located," Spiegel reported.
Spiegel said that security officials tracked the calls to a building complex at the NATO headquarters used by NSA experts.
(Read More: How Long Can Snowden Stay at the Moscow Airport)
Documents provided by Snowden to The Guardian newspaper that were published earlier this month revealed that NSA spying was much wider than previously thought. Under a program known as Prism, the agency allegedly tapped directly into the servers of nine internet firms including Facebook, Google, Microsoft and Yahoo to track online communication.
Snowden remains stuck in limbo, nearly a month after the revelations first emerged. He is believed to be in the transit area of Moscow airport. After fleeing from Hong Kong, he is said to be contemplating seeking asylum in Ecuador, Venezuela or Cuba, Reuters reported on Saturday.
The U.S., which has charged Snowden with espionage, has put heavy pressure on Russia and any country willing to take him in.