ATHENS, July 3 (Angeliki Koutantou/Reuters) - Greece offered to ferry humanitarian aid to the Palestinian Authority on Sunday after banning a pro-Palestinian flotilla bound for Gaza from departing from its ports and arresting the captain of one of the boats.

The convoy of cargo and passenger boats carrying volunteers from countries including the United States, Canada, France and Spain was due to depart at least a week ago.

In a telephone call with Palestinian President Mahmoud Abbas on Sunday, Greek Prime Minister George Papandreou offered to ferry the humanitarian aid, mainly food and medicines, to Gaza in cooperation with the United Nations.

President Mahmoud Abbas considered the proposal positive and expressed his support, said a statement by the prime minister's office. There might be additional aid from Greece in addition to the supplies that would have gone on the flotilla.

Just over a year after nine activists were killed when Israeli marines stormed another pro-Palestinian flotilla, Greek authorities said on Friday that ships destined for Gaza were prohibited from leaving Greek ports -- even though they would be sailing into international waters.

In a separate statement released on Sunday, Greece's Foreign Ministry said the Greek ban was imposed because of safety concerns and in line with a U.N. request and EU
recommendations.

U.N. Secretary General Ban Ki-moon spoke by telephone with Greece's foreign minister, Stavros Lambrinidis, on Sunday, according to a U.N. statement.

Ban supported the initiative to use Greek vessels to route humanitarian aid to Gaza through regular existing channels, the statement said, adding that in Ban's view it could help to reduce tensions in the region and ensure much-needed aid is delivered to those who need it in Gaza.

U.N. DISCOURAGES FLOTILLAS

At the United Nations in New York, the Quartet of Middle East peace negotiators had issued a statement saying such flotillas should be discouraged.

The ministry reiterated Greece's view that Israel's blockade of Gaza should be lifted and humanitarian conditions in the area improved.

Greece reiterates its willingness and 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 secretary-general of the United Nations, it said.

Israel says its blockade of Gaza is aimed at stopping weapons from reaching the enclave's rulers, Hamas -- an Islamist group that is branded a terrorist organization by the
West.

A boat ferrying mostly American activists, The Audacity of Hope, tried to leave Perama port near Piraeus on Saturday.

It was turned back by the Greek coastguard, which said the captain, John Klusmire, 60, had been arrested and would appear before a court on Tuesday to face felony charges.

Greek coastguard spokesman George Economou said the U.S. captain had been arrested for having no permission to sail and for exposing passengers to possible danger. The captain will appear before an investigator on Tuesday, he said.

In a news briefing on Saturday, organizers accused the Greek government of being forced to impose the ban because the U.S. government did not want the flotilla to set sail.

Organizers said 10 ships including The Audacity of Hope, with around 350 volunteers, were ready to take part.

But most vessels were blocked in ports in Crete, Corfu or Athens, and Twitter updates from journalists who had planned to join the trip suggested some were leaving out of frustration.

(Reporting by Angeliki Koutantou, additional reporting by Megan Davies at the United Nations; editing by Peter Millership and Todd Eastham)