An anti-tank shell was fired at the US embassy early Friday, striking the front of the building but causing no injuries. Greece's Public Order Minister said the blast was probably an act of domestic terrorism _ raising fears of resurgent violence by far-left Greek militants. "It is very likely that this is the work of a domestic group," Vyron Polydoras said. "We believe this effort to revive terrorism is deplorable and will not succeed." He said Greece "strongly condemns" the attack on the heavily guarded building - the first major attack against a US target in Greece in more than a decade. "We believe it is a symbolic act," Polydoras said. "It is an attempt to disrupt our country's international relations." Polydoras said police were examining the authenticity of phone calls to a private security company claiming responsibility on behalf of a militant left-wing group. "There are one or two telephone calls, from unknown callers, who claimed that the Revolutionary Struggle assumes responsibility," Polydoras said. "We cannot rule out that they were genuine." Revolutionary Struggle claimed responsibility for a May 2006 bomb attack on Culture Minister Giorgos Voulgarakis, in which nobody was injured. Police cordoned off streets around the building after the explosion shortly before 6 a.m. (0400 GMT). The shell struck the third floor - narrowly missing the embassy emblem - and smashed glass in nearby buildings. Investigators were examining what they believed was the device used to fire the shell from a construction site near the embassy. Embassy officials confirmed that an explosion had taken place and said that no one had been injured. US ambassador Charles Ries said the damage was "not extensive." "There can be no justification for such a senseless act of violence. ... The embassy was occupied at the time (but) nobody was hurt," he said. Ries said there had been no warning of the attack. "We're treating it as a very serious attack. We will determine what it is when we know (more)," he said. Authorities were searching apartment buildings near the US Embassy and a nearby hospital for evidence. "At 5:58 a.m. an explosion was caused by a self-propelling shell, causing minor damage at the front of the building," a police statement said. Greek Foreign Minister Dora Bakoyannis visited the embassy after the blast. "I came here to express the solidarity of the Greek people following this deplorable action," she said. "Such actions in the past have had a very heavy cost for the country. ... The Greek government is determined to undertake every effort to not allow such phenomena to be repeated in the future." A US State Department spokesman confirmed that the attack caused no injuries. "There are no injuries or casualties of any kind," Kurtis Cooper said. "Police have responded. As a result the embassy will be closed today, Friday." Polydoras, the Public Order Minister, said police would set up a special task force on the attack, headed by a former counterterrorism chief who eradicated the far-left November 17 group in 2002. The group was blamed for killing 23 people - including US, British and Turkish officials - and dozens of bomb attacks. In 2003, a special court gave multiple life sentences to November 17's leader, chief assassin and three other members. Lesser sentences were given to 10 others. November 17 carried out a similar rocket attack against the US embassy in 1996, causing minor damage and no injuries. Several obscure militant groups have appeared since the November 17 members were arrested. Radical groups Revolutionary Struggle and Popular Revolutionary Action were blamed for the bombings of three government ministries in 2005.