Anti-terrorism police investigating a bomb threat searched apartments at several buildings around the U.S. embassy in Athens Wednesday and briefly closed a busy road in front of the embassy building, authorities said.

Eleven people were being questioned by police after the search, but no explosives were found.

The Public Order Ministry said it had received information late Tuesday from a foreign national of a possible plan to attack the embassy.

"From the investigation so far, the information (about the threat) appears to be inaccurate," the ministry said in a statement. "Despite that, all the necessary police measures were taken as a precaution."

A senior police official, who asked not to be named, said all 11 people being questioned were Iraqi immigrants, and that the man who made the claim was also Iraqi.

U.S. ambassador Charles Ries was notified of the police action by Public Order Minister Vyron Polydoras, the ministry said.

Several roads behind the embassy building remained closed. Searches were being conducted in at least three buildings.

Police had originally told drivers in the area that the operation was an exercise. The rush-hour road closures caused traffic congestion across central Athens.

Security around the U.S. Embassy in central Athens has been heightened after attackers struck the building with a rocket-propelled grenade on Jan. 12, causing damage but no injuries.

A far-left Greek group, Revolutionary Struggle, later claimed responsibility for that attack, saying their action was in response to U.S. military involvement in Iraq, Afghanistan and Somalia.

Last month, the Greek and U.S. governments offered separate million-dollar rewards for information leading to the prosecution of members of the group.