ATHENS, Greece-A bomb exploded outside Greece's Ministry of Environment and Public Works on Sunday, causing slight damage, officials said. No injuries were reported.

The bomb went off at 4:50 p.m. (1450 GMT) and smashed some windows at the ministry. Twenty-five minutes earlier, an unknown man had called daily newspaper Eleftherotypia to warn of the impending explosion.

"A bomb will go off in 25 minutes," he said. "This is not a hoax, inform people immediately."

The ministry's general secretary for Public Works and a few employees working at the time were evacuated by police.

Police officials, who found the remains of the bomb at a nearby trash can, said its makeup was similar to that of other bombs detonated by terrorist organizations "Popular Revolutionary Action" and "Revolutionary Struggle."

The latter has targeted ministries before and on January 12, 2007 fired a rocket at the U.S. Embassy. Police said they expect a statement by either organization in coming days.

"The government unequivocally condemns actions such as today's bomb attack on the Ministry of Environment and Public Works, actions that are aimed against democracy itself," government spokesman Theodoros Roussopoulos said.