EU spokesman Dominique-George Marro said listening devices also were found in offices used by other EU nations. He declined to say which offices or how many devices.
The bugging equipment was found a few days ago, Marro said, but investigators wanted to keep the discovery quiet to stand a better chance of catching the culprits. France's Le Figaro newspaper broke the story Wednesday.
"We found anomalies in the telephone lines" during regular security checks, Marro said.
He said only a small number of lines had been affected in the sprawling glass and marble Justus Lipsius building in central Brussels.
Leaders of the 15 EU nations are scheduled to meet Thursday and Friday at the building for their annual spring summit, during which they are also expected to discuss the Iraqi crisis.
The building houses delegations from all EU countries, the EU's military staff, and the offices of Javier Solana, the high representative for foreign and security policy. Earlier this week it was the venue for regular talks among foreign and finance ministers.
"There is an urgent interest in clearing this up," German Interior Ministry spokesman Rainer Lingenthal said. "We still hope to find those responsible."
Greek Foreign Minister George Papandreou, whose country holds the EU's rotating presidency, said he was informed of the bugging Wednesday morning.
"The first thing I can do is to condemn this act," he told a news conference.
"The details are still being investigated," Papandreou said. "Once we get the results, obviously you will be informed. ... We will find out soon and take the appropriate measures."
The short report in Le Figaro said Belgian police had identified the bugs as American, but EU officials did not confirm this.
"At this point we cannot say who planted these bugs," said Solana's spokeswoman, Cristina Gallach.
"We have received no communication about the investigation from the EU," said a spokesman for the U.S. mission to the EU, who asked not to be named.
Gallach added that no devices were found on the phones of Solana or at the EU's military wing, which is housed in the same building.
Two years ago the European Parliament probed allegations that United States intelligence agencies were eavesdropping on phone calls and e-mail traffic in hopes of intercepting sensitive government and business communications. The lawmakers concluded that the spy network, dubbed Echelon, existed and warned EU nations to step up security measures.
EU Executive Commission spokesman Reijo Kemppinen said Wednesday that security precautions at the EU's headquarters nearby the Justus Lipsius building had not been compromised by the latest discoveries.
By Constant Brand