Israel has announced that its navy will attempt to stop two boats carrying pro-Palestinian activists bound for the Gaza Strip, in the latest attempt by activists to break the four-year Israeli blockade against the territory.

The Israeli military spokesman's office said the country's navy was "prepared to contact" the vessels and had "completed the necessary preparations in order to prevent them from reaching the Gaza Strip".

The Canadian boat Tahrir and the Irish boat MV Saoirse left the port of Fethiye in southwest Turkey on Wednesday after Turkish authorities gave them permission to sail to the Greek island of Rhodes.

Al Jazeera's Casey Kauffman, on board one of the ships, said that in total it would be a 50-hour journey, and they were currently one-fifth of the way there.

"Everyone on the boat wants to get to Gaza," he said, adding that while the activists are prepared for the possibility of an Israeli interception, the initiative will not be wasted.

"It will still bring attention to the situation in Gaza, and the blockade of the Gaza Strip." 

Lieutenant-Colonel Avital Leibovich, an Israeli military official, would not say how the boats might be stopped, saying only "we will have to assess and see if we are facing violent passengers."

Describing their journey as a "provocation", she said Israel would offer to unload any aid supplies on board and deliver them to Gaza.

Sailing under the flag of the Comoros Islands, the Tahrir is carrying six activists, a captain and five journalists.

The Saoirse - sailing under the US flag - has 12 Irish nationals on board, none of whom are journalists.

'Support from Turkish society'

David Heap, a member of the steering committee on board the Tahrir, told Al Jazeera that the activists chose to leave from Fethiye because of the strained relations between Turkey and Israel.

"The Turkish government has been creating more distance from Israel diplomatically and we know there is support from Turkish society for what we are doing.

"Our judgment was that the Turkish state would not interfere with us if we didn’t make too much of a public issue of our plan to depart from there," Heap told Al Jazeera.

It will take at least a couple of days before the boats reach the Palestinian waters of the Gaza Strip, where they expect to be approached by the Israeli navy.

"We have some distance to cover between where we are now and Palestinian territorial waters of Gaza.

"Obviously we are going to avoid going through Israeli territorial waters.

"Our plan is to go directly from international waters into the territorial waters of Gaza - within a couple of days.

The activists say the new attempt the break the siege on the Gaza Strip is part of a campaign they call "freedom waves", implying that more such efforts will follow.

Both ships were part of previous attempts to break the siege on the Gaza Strip that was stalled when the Greek government refused to let a flotilla leave from its shores in July this year.

The Tahrir, the larger ship of the two, was intercepted by the Greek coast guard with more than 30 pro-Palestinian activists onboard.

Two of them were detained for defying Greece's ban on setting sail to Gaza. The vessel was stopped about 10 minutes after it left port on the island of Crete.

The Irish boat allegedly suffered damage when it was sabotaged while waiting to join the flotilla from Turkish waters. The ship has since been repaired and kept in dry-dock in Turkey.