Greece has banned all ships in the Gaza-bound "freedom flotilla" from leaving port, dealing a further blow to activists trying to break Israel's blockade on the Palestinian territory.

Greek authorities said an international group of vessels planning to sail from its ports and deliver aid to the Gazan population would be stopped, a move that lends the support of the Prime Minister, George Papandreou's administration to Israel's contentious four-year naval blockade of the Gaza Strip.

An American boat participating in the flotilla was forced to return to shore after it tried to defy the ban and set sail from Athens on Friday. The Audacity of Hope was turned back by the Greek coastguard. Passengers claim Greek commandos pointed machine guns at those on board to stop the boat reaching open water.

Campaigners accused Israel of "outsourcing" its blockade to Greece. "Greece sold its body to the banks and its soul to Israel and the United States," flotilla activist Dror Feiler told Israeli news outlet Ynet. "I don't think – I know – that Israel and US pressure caused this." Hamas also condemned the Greek decision, describing it as "inhumane" and "contrary to international regulations and norms".

The Greek announcement is the latest in a series of setbacks for the organisers of this summer's flotilla, which comes just over a year after a similar mission ended in the deaths of nine activists following the boarding of their boat by Israeli military forces. Participants claim that two of the 10 ships in the flotilla had been sabotaged by Israeli agents – a claim Israel dismissed as "ridiculous".

The Israeli government has described the flotilla as an act of anti-Israeli provocation rather than an attempt to convey much-needed aid to Gaza's 1.6 million inhabitants, who have lived under an economic blockade since Hamas took control of the territory in June 2007.

However, Israel has been embarrassed by the release of an anti-flotilla video which was later exposed as an elaborate hoax.