Greece has offered to transport aid to Gaza using its own vessels under UN supervision, after it detained several ships from the Freedom Flotilla carrying aid and pro-Palestinian activists.

Members of the Dutch-Italian boat issued an open letter to Greek prime minister George Papandreou expressing outrage over his government's decision to close the Greek ports to their humanitarian expedition.

The Greek coastguard at the Piraeus port in Athens arrested the crew of one ship after it attempted to sail for Gaza without permission.

Activists have accused Israel of putting political pressure on Greece to prevent the vessels from leaving shore.

Israel insists it has had no hand in the flotilla's detention but warned it would not allow the ships to break its blockade of Hamas-controlled Gaza.

"[Greece] proposes to undertake the task of transporting the humanitarian aid, with Greek vessels or other appropriate means, through the existing channels" as requested by the UN, said a Greek foreign ministry statement.

Greece is "concerned primarily with the safety of human life", the ministry statement insisted.

Prime minister George Papandreou spoke to the head of the Palestinian Authority Mahmud Abbas on Sunday to discuss the offer to transport aid, a government statement said.

While stressing it would be done "in cooperation with the UN and the competent authorities", Greece said the initiative would involve "constant contact with the Palestinian Authority".

"[Mr Abbas] considered the proposal positive and expressed his support," it added.

Campaigners with the flotilla denounced Mr Papandreou after a Greek coastguard vessel carrying armed, masked men intercepted the US boat Audacity of Hope as it tried to break the Greek ban on Friday.

On Sunday, they expressed concern for the plight of the captain of the boat, John Klusmer, who was arrested and charged with leaving the port without permission and endangering the lives of passengers.

US activists said his jail conditions were shocking and complained that neither he nor the US passengers had received any help from the US embassy.

Members of the US boat to Gaza began an open-ended fast outside the embassy "calling on the US government to defend our right to sail out of Greece".

About 100 of the activists, middle-aged Americans and Europeans, also staged a protest in Athens's parliament square on Sunday evening. The square was the scene of riots earlier this week over the government's austerity cuts.

- ABC/AFP