Barcelona, Spain - Fierce Mediterranean winds forced a Gaza-bound flotilla carrying humanitarian aid and hundreds of pro-Palestinian activists, including Greta Thunberg, to return to Barcelona, organizers said on Monday.

Around 20 vessels left the Spanish city on Sunday aiming to "open a humanitarian corridor and end the ongoing genocide of the Palestinian people," said the Global Sumud Flotilla – "sumud" being the Arabic term for "resilience."

But "due to unsafe weather conditions, we conducted a sea trial and then returned to port to allow the storm to pass," the organization said in a statement, without specifying when exactly the boats returned to Barcelona.

"This meant delaying our departure to avoid risking complications with the smaller boats," it added, citing gusts that exceeded 34 miles per hour.

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"We made this decision to prioritize the safety and well-being of all participants and to safeguard the success of our mission."

Spanish media reported that the organizers would meet to decide whether to resume the expedition later on Monday.

Among the activists from dozens of countries were Thunberg, actors Liam Cunningham of Ireland and Eduard Fernandez of Spain, as well as European lawmakers and public figures, including former Barcelona mayor Ada Colau.

The flotilla is expected to arrive in Gaza in mid-September and comes after Israel blocked two activist attempts to deliver aid to the devastated Palestinian territory by ship in June and July.

The United Nations has declared a famine in Gaza, warning that 500,000 people face "catastrophic" conditions.

Israel has killed at least 63,459 people in Gaza since October 2023, according to figures from the territory's health ministry, though the true number is believed to be far higher.