The 11 Australians were among 426 people taking part in a 54-vessel flotilla from 39 countries aiming to challenge a naval blockade of Gaza.

The organisers of a flotilla of ships trying to break Israel's maritime blockade of Gaza say Israel's navy is about to begin intercepting the group off the coast of Cyprus.

The wellbeing of 11 Australian activists is unknown after their Gaza-bound flotilla was intercepted overnight.