Israel's military chief ordered the navy Friday to intercept two Gaza-bound boats carrying pro-Palestinian activists who were challenging Israel's maritime blockade of Hamas-ruled Gaza, a statement from Israel's military said.

Lieutenant General Benny Gantz issued the instructions after all attempts to contact activists aboard the Canadian and Irish vessels had failed, the statement said.

The Canadian Tahrir and Irish Saoirse vessels were in international waters when the order was given and there were no immediate reports of violence between the 27 people on board and an Israeli troops.

In May 2010, Israeli commandos boarded the Turkish Mavi Marmara aid vessel to enforce its naval blockade of the Palestinian enclave, and killed nine Turks in clashes with activists, some of them armed with clubs and knives.

Israel spurned Ankara's demand for an apology over the incident and it expelled the Israeli ambassador two months ago.

Carrying medical supplies, the Tahrir and Saoirse had set sail from Turkey Wednesday.

The activists on board came from Australia, Canada, Ireland and the United States, and included Palestinians and at least one Arab citizen of Israel, organizers said.

The two boats had continued sailing towards Gaza, ignoring instructions to turn around or unload their supplies in Israel or neighboring Egypt, Israeli military officials said.

After announcing the interception order, the military said the goods might be unloaded at Israel's Ashdod port and taken to Gaza via land crossings.

Israel has blockaded Gaza since Islamist group Hamas seized control of the territory in 2007, arguing that the shutdown was necessary to prevent weapons smuggling.

Pro-Palestinian groups behind the latest sailing condemn the blockade as illegal and inhumane.

A U.N. report on Israel's interception of the 2010 Turkish ship said the blockade was a legitimate security measure, adding that its implementation complied with the requirements of international law.

Paul Murphy, a socialist member of the European Parliament on board one of the ships wrote in a blog posted earlier on the Internet that the mission was in response to the call from people within Gaza to try to break the siege they suffer under.

Some of the activists, who dubbed their mission Freedom Waves, had participated in a thwarted seaborne attempt in June to reach Gaza, which was blocked from setting sail from Greece.

(Additional reporting by Nidal al-Mughrabi; Writing by Allyn Fisher-Ilan and Jeffrey Heller; editing by Crispian Balmer)