A ship carrying humanitarian aid to Gaza was attacked from the air near Malta, causing fire onboard, the Freedom Flotilla Coalition said on Friday.
The international NGO said the ship Conscience was attacked by a drone in international waters shortly after midnight.
"Armed drones attacked the front of an unarmed civilian vessel twice, causing a fire and a substantial breach in the hull," the NGO said in a statement.
It also posted a video of the fire and apparent damage online.
A spokesperson for the Freedom Flotilla Coalition told the AFP news agency that they "suspect" Israel was behind the attack, but "cannot confirm" with certainty.
Israel is "the primary entity interested in keeping us and any aid out of Gaza," said spokesperson Hay Sha Wiya.
The Freedom Flotilla called for "Israeli ambassadors" to be summoned and answer for the ongoing Gaza blockade "and the bombing of our civilian vessel in international waters."
Israel did not immediately comment on the incident.
A previous Freedom Flotilla convoy ended in bloodshed in 2010, when Israeli forces stormed the Mavi Marmara vessel, killing 10 people and injuring 28 more.
Greta Thunberg meant to board the ship
The Maltese government said a tug boat had been dispatched and brought the fire under control by early Friday morning.
The vessel and the crew were secure, and the Conscience crew refused to board the tug, according to the government statement.

No injuries were reported in the incident. The organization claimed 30 people were onboard the ship at the time of the attack, while Maltese officials spoke of 12 crew members and four civilians.
The discrepancy might be due to plans for more people, including Swedish activist Greta Thunberg, to board from another ship.
"I was part of the group who was supposed to board that boat today to continue the voyage towards Gaza, which is one of many attempts to open up a humanitarian corridor and to do our part to keep trying to break Israel's illegal siege on Gaza," Thunberg told Reuters.
UN says 'people are killing each other over water' in Gaza
The attack on the humanitarian ship came as the United Nations said on Friday that food supplies in Gaza "have now mainly run out."
Following the collapse of a fragile ceasefire in March, Israel renewed its strikes on Gaza and reestablished an aid blockade for the region.
UN decries Israeli blockade of aid supplies to Gaza
Speaking to reporters in Geneva via video link from Gaza City, UN humanitarian spokesperson Olga Cherevko said community kitchens were shutting down and more people were going hungry. Hospitals were running out of blood while casualties continue to mount, and water access was becoming impossible, she added.
"In fact, as I speak to you, just downstairs from this building people are fighting for water. There's a water truck that has just arrived, and people are killing each other over water," the OCHA spokesperson said.
Cherevko also criticized decision-makers who "have watched in silence the endless scenes of bloodied children, of severed limbs, of grieving parents move swiftly across their screens, month, after month, after month."
At the same time, the International Committee of the Red Cross called "immediate action."
"The humanitarian response in Gaza is on the verge of total collapse," they said.
Edited by: Zac Crellin