Israel will not permit a flotilla of pro-Palestinian activists who set sail from Turkey on Wednesday to reach their destination, the Gaza Strip, a government official told Reuters.

They will not be allowed to breach the blockade, the official said, speaking on condition of anonymity. He would not say what measures would be taken to stop the two ships, but confirmed Israel knew they were heading for Gaza's shores.

The blockade is on, and no exceptions will be made, he said. We are monitoring the situation. If indeed they are going to breach the blockade we will take whatever measures will be necessary, the official added.

Israel has blockaded Gaza since Hamas Islamists seized control of the territory in 2007, after routing Western-backed Palestinian President Mahmoud Abbas. Israel permits humanitarian aid and supplies to reach the territory through a land crossing, and Gaza also shares a border with Egypt.

The Israeli military had no immediate comment on the ships heading for Gaza.

Israeli commandos killed nine Turkish nationals on one ship in a Gaza-bound flotilla last year when the activists fought them with clubs and knives as the commandos tried to seize control of the ship to enforce Israel's blockade.

The incident badly damaged ties between Israel and Turkey, which reached a crisis point two months ago when Ankara expelled the Israeli ambassador after Israel rejected Turkey's request for an apology for the flotilla deaths.

(Writing by Allyn Fisher-Ilan; Editing by Tim Pearce)