A flotilla carrying medication is bound for the Gaza Strip after quietly departing a Mediterranean port, activists announced on Wednesday as Israel's army said it was "fully prepared" to stop it.

The Canadian Tahrir and the Irish Saoirse are carrying 27 people from nine countries, organizers said. The ships are carrying a symbolic cargo of $30,000 in medication.

The 27 activists on board the vessels are drawn from Australia, Canada, Ireland and the United States, and included Palestinians.

The ships were in international waters Wednesday and hoping to reach Gaza by Friday. They set sail from Fethiye, Turkey, according to activists on board the two ships.

Israel authorities vowed to stop the flotilla, which they view as a provocation.

"The Israeli Navy has completed the necessary preparations in order to prevent them from reaching the (Gaza) Strip," said Lt.-Col. Avital Leibovich, an Israeli army spokeswoman, on Twitter.

Authorities are "fully prepared to guard the safety of Israel and its maritime border," she says.

The boats "will not be allowed to breach the blockade," another official told Reuters, speaking on condition of anonymity. "The blockade is on, and no exceptions will be made," he said.

Israel "will take whatever measures will be necessary."

The "Freedom Waves to Gaza" kept the voyage secret until now due to accusations in July that Israel tried to sabotage a previous effort, the activists said in a statement Wednesday.

"Israel's actions in extending its illegal blockade all the way to shores of Greece required that we organize this new effort quietly," the Canadian Boat to Gaza wrote on its Twitter account.

US-based news channel Democracy Now posted footage of activists on board the Tahrir.

"Israel has caged Palestinians in Gaza and the West Bank, prohibiting physical contact between us. We want to break the siege Israel has imposed on our people," said Majd Kayyal, a student from Haifa.

"The fact that we’re in international waters is already a victory for the movement. Israel's siege of Gaza is untenable and it's a moral responsibility to put an end to this injustice."

The latest challenge to Israel's blockade of the Palestinian enclave comes during a period of heightened tension in the eastern Mediterranean.

Turkey was angered by Israel's refusal to apologize for the killing of nine Turks when Israeli commandos boarded the Mavi Marmara aid vessel 2010.

It was also disappointed by the conclusions released in August of a UN inquiry into the incident, and has 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, having 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.

Activists tried to send another flotilla to Gaza in June, but all nine vessels were stopped.

They were intercepted or stopped from leaving port, suffered suspicious damage before sailing, or pulled out amid speculation that their organizers had succumbed to political pressure.