The international flotilla trying to deliver aid to Gaza said on Wednesday that several of its boats were approached by unidentified vessels, some navigating without lights.
The Global Sumud Flotilla said in post on Telegram that the vessels have departed, and participants implemented security protocols in anticipation of a possible interception.
The flotilla of 45 vessels carrying activists and politicians, including Swedish campaigner Greta Thunberg and Nelson Mandela's grandson Mandla Mandela, departed Spain last month with the aim of breaking the blockade on Gaza, where the UN has said there is a famine.
After departing Spain, the flotilla stopped in Tunisia for 10 days before resuming its journey on 15 September.
Israel has already blocked two attempts by activists to deliver aid by ship to Gaza, in June and July.
In June, 12 activists on board the sailboat Madleen, including Thunberg, were intercepted by Israeli forces 185 kilometers (115 miles) west of Gaza.
At 0530 GMT, the Global Sumud Flotilla reported that it was in the Mediterranean north of the Egyptian coast and approaching the 120 nautical mile mark (approximately 220 kilometers) from Palestinian territory.
"We sail on undeterred by Israeli threats and tactics of intimidation," the flotilla added in its statement on X.