Israel has intercepted ships carrying publicity aid bound for Gaza in international waters near Greece, flotilla organisers said on Thursday, decrying the move as an "escalation of Israel's impunity".
A second flotilla sailed from the Spanish port of Barcelona on April 12, aiming to try to break the Israeli blockade.
The vessels were seized by Israel hundreds of miles from Gaza, said the organisers, Global Sumud Flotilla.
"This is piracy," the activist group said in a statement.
Israel's UN envoy Danny Danon said the flotilla, "was stopped before reaching our area".
In a posting on social media, he added, "Our brave IDF soldiers are acting with professionalism and determination dealing with a group of delusional attention-seeking agitators," referring to the Israeli military.
Israel's military halted a previous flotilla assembled by the same organisation last October in an attempt to reach blockaded Gaza, arresting controversial Swedish campaigner Greta Thunberg and more than 450 activists.
Israel, which controls all access to the Gaza Strip, denies withholding supplies for its residents, numbering more than two million.
(Reporting by Menna Alaa El Din and Muhammad Al Gebaly; Additional Reporting by Hatem Maher; Editing by Chris Reese, Clarence Fernandez and Baird Maritime)