JERUSALEM, Nov 2 (Allyn Fisher-Ilan/Reuters) - Pro-Palestinian activists set sail for the Gaza Strip from Turkey aboard two yachts on Wednesday and the Israeli military said it would not permit them to breach its blockade of the Islamist-ruled territory.

The 27 activists on board the Canadian and Irish vessels came from Australia, Canada, Ireland and the United States, and included Palestinians and at least one Israeli Arab citizen.

Jane Hirschmann, a New York-based organizer of the flotilla dubbed by a coalition of activists the Freedom Waves to Gaza, said by telephone that the ships were out in the Mediterranean and headed toward Gaza and suggested they hoped to reach the Palestinian enclave by Friday.

The flotilla, carrying medical supplies, had set sail unannounced in the morning so as to avoid any attempts to stop them, Hirschmann said. In June they were blocked from setting sail from Greece.

We wanted to make sure we could reach international waters. We didn't want any government to intervene, she said. The activists aim to challenge Israel's naval blockade of the Hamas-ruled territory.

Israeli military officials said they were tracking the boats and would bar them from reaching Gaza to enforce a blockade Israel regards as necessary to prevent weapons from being smuggled to gunmen in the enclave.

In May 2010, Israeli commandos boarded the Mavi Marmara aid vessel to enforce the Gaza blockade and clashed with activists. Nine Turks were killed in the incident.

Lieutenant-Colonel Avital Leibovich, a military spokeswoman, would not say how the boats might be stopped.

We will have to assess and see if we are facing violent passengers, Leibovich briefed reporters by telephone, adding that Israel saw the flotilla as a provocation.

She would not say whether Israel knew whether any of the passengers were armed. An emailed statement from one of the groups involved said their effort would be non-violent.

Israel would offer to unload any aid supplies on board and deliver them to Gaza, Leibovich said.

TENSE TIMES IN THE MEDITERRANEAN

The latest challenge to Israel's embargo of Gaza, in force since Hamas seized control of the territory in 2007 from Western-backed Palestinian President Mahmoud Abbas's forces, comes during a period of tension in the eastern Mediterranean.

Turkey was angered by Israel's refusal to apologize for the killing of the nine Turks in May 2010. Two months ago Ankara expelled the Israeli ambassador.

Also disappointing Turkey were the conclusions in August of a United Nations inquiry into the incident. Ankara has since increased its naval presence in the Eastern Mediterranean.

A Turkish foreign ministry official confirmed that two boats had set sail from Turkey's southern coast and that they had asked for permission to go to the Greek island of Rhodes.

The official said the vessels were not Turkish-flagged, had no Turkish passengers and the captains were not Turkish.

Paul Murphy, a socialist member of the European Parliament, was among the activists on board the Irish ship Saoirse.

This ship to Gaza is another response to the call from people within Gaza to try to break the siege they suffer under, Murphy wrote in a blog posted on the Internet.

Ann Wright, described in a statement by the group Ustogaza as a former U.S. army officer and diplomat, and on board the Canadian boat, Tahrir, was quoted as saying:

We carry inspiration from the Arab Spring and the worldwide Occupy movements that are demanding freedom and justice. Where governments fail, civil society must act. As Americans we are fed up with our government's unquestioning support of Israel.

Israel permits humanitarian aid, food and other supplies to enter Gaza for its 1.5 million people many of them impoverished refugees, via land crossings it closely monitors. Gaza also has a border with Egypt over which goods are imported. (Additional reporting by Simon Cameron-Moore in Istanbul, Nidal al-Mughrabi in Gaza; Editing by Robert Woodward)