A makeshift bomb has exploded outside an Athens bank and a Greek fringe group has claimed responsibility for three blasts in the city last week, saying they were to protest heavy security measures for the Olympics.

Thursday's early morning blast outside a branch of Alpha Bank caused minor damage and no casualties, but will raise more concern about the safety of the Games this summer.

The group "Revolutionary Struggle" told the newspaper To Pontiki that last week's bombs which damaged a police station but caused no casualties were a response to Olympic security plans.

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"With regard to the Olympic Games we say that Greece's transformation into a fortress, NATO's involvement, the presence and activities of foreign intelligence units show clearly that (the Olympics) are not a festival like Games organisers say, but it's a war," it said in a statement sent to the newspaper.

"All members representing the international capital, global mercenary killers and state officials, as well as well-off western Olympic tourists planning to be here, are not welcome," it said.

Greek Prime Minister Costas Karamanlis has said the recent blasts were isolated domestic incidents which did not affect the security of the Games.

"We are preparing well for the safety of the Olympic Games in order to avoid and prevent any potential threat," Karamanlis told reporters on Thursday.

BANKS TARGETED

Thursday's blast was triggered by gas canisters. Bomb squads also located a bag containing another similar home-made device outside an HSBC bank, across the road that did not go off.

The claim and latest blast came as Greek security forces started a crucial three-day Olympics safety drill three months before the start of the Games.

"Olympic Guardian II", the eighth Olympic safety drill, involves eight key ministries as well as Games organisers and other services.

"The drill's difficulty scale is escalating every day and it tests our responses to major terrorist attacks," police spokesman Lefteris Eceonomou told Reuters.

Greece is investing about one billion euro on security, several times the amount spent in the Sydney 2000 Games and will get NATO assistance in air and sea patrols.

In 2002, the government claimed victory over what was then considered the main Olympics security threat, when it arrested leading members of the November 17 leftist guerrilla group.

"Revolutionary Struggle" placed a bomb outside Athens courts in September, injuring a police officer, and another outside a Citibank branch in March. Police have not made any arrests and have said they know little about the group.

In a letter left in a garbage bin near the offices of To Pontiki, the group laid out a lengthy anti-capitalist manifesto which it said explained the reasons for its armed struggle.

Late on Wednesday, Greek police briefly detained five British and German journalists who entered two main Olympic venues without permits, officials said.