Earlier on Monday, Israel's foreign ministry had said on X it "will not allow any breach of the lawful naval blockade on Gaza".
Ships from the Global Sumud Flotilla had set sail for a third time on Thursday from southern Turkey, after earlier attempts to deliver aid to Gaza were intercepted by Israel in international waters.
Live video showed military vessels approaching the vessels on Monday.
"Military vessels are currently intercepting our fleet and (Israeli) forces are boarding the first of our boats in broad daylight," the Global Sumud Flotilla initially said on X.
"We demand safe passage for our legal, non-violent humanitarian mission."
The group said 10 boats had been intercepted and contact lost with 23 of the 54 vessels in the flotilla, naming some two dozen Turks among those on the intercepted vessels, some 250 nautical miles (463 km) from Gaza.
It said there were 426 people taking part in the flotilla from 39 countries.
Israel's foreign ministry also called on "all participants in this provocation to change course and turn back immediately".
The previous flotilla departed from Spain on April 12. But Israeli forces intercepted vessels in that group, taking more than 100 pro-Palestinian activists to Crete and detaining two others in Israel.
Last October, Israel's military halted another flotilla assembled by the same organisation, arresting Swedish activist Greta Thunberg and more than 450 participants.
Palestinians and international aid bodies, along with Turkey and a number of other countries, say supplies reaching Gaza are still insufficient, despite a ceasefire reached in October that included guarantees of increased aid.
Most of Gaza's more than 2 million people have been displaced, many now living in bombed-out homes and makeshift tents pitched on open ground, roadsides, or atop the ruins of destroyed buildings.
Israel, which controls all access to the Gaza Strip, denies withholding supplies for its residents. Its foreign ministry said more than 1.58 million tons of humanitarian aid and thousands of tons of medical supplies have entered Gaza since October 2025.
