The Gaza-bound humanitarian mission Global Sumud Flotilla has said that all vessels in its aid convoy have been seized by Israel.
"We are awaiting more information about their illegal abduction," organisers of the flotilla said in a statement on Telegram on Tuesday.
Vowing to continue its mission for Palestine, it called on people to “demand safe passage".
"Send emails to your foreign ministers," it added.
Meanwhile, the UN said it is "very concerned" about the safety of activists aboard.
"We're very concerned about the safety of all those on board. They need to be protected, and they need to be kept safe," said UN spokesperson Stephane Dujarric at a news conference, adding that "international law on the high seas needs to be respected."
Asked whether Secretary-General Antonio Guterres would insist that the apprehension of flotilla activists constituted a violation of international law, Dujarric said, "I don't think we have all the details of how this was done, but it doesn't look like this was done in full respect of international law."
Dujarric also renewed calls for Israel to ease restrictions on humanitarian aid entering Gaza.
"Let's remember the best way to get humanitarian aid in is through official channels, and for that to happen at greater volume, we need to see Israel remove a number of the impediments and barriers that have been in place, which don't allow us to bring the aid that we need in," he said.
Dujarric highlighted critical shortages of spare parts for generators and pumping stations, mounting solid waste, and a lack of materials needed to rebuild shelters at scale in the besieged Palestinian enclave.
Earlier in the day, the flotilla said that Israeli military vessels were continuing to attack and board its humanitarian aid fleet in international waters as the convoy headed toward Gaza.
The developments came as the Israeli army launched attacks Monday against the aid convoy in international waters.
The flotilla, comprising 54 boats, departed on Thursday from the Turkish district of Marmaris in a new attempt to break the illegal Israeli blockade imposed on Gaza since 2007.
This was not the first such incident involving the flotilla.
In late April, the Israeli army attacked flotilla boats in international waters off the Greek island of Crete. The convoy at the time included 345 participants from 39 countries, including Turkish citizens.
