A ship carrying humanitarian aid and activists heading for Gaza was bombed by drones while in international waters off Malta earlier today, The Freedom Flotilla Coalition, an international NGO said, Reuters reports.

It uploaded video footage showing a fire on one of its ships, but did not indicate who could have been responsible for the attack. There was also no immediate indication whether anyone had been hurt.

“The drone attack appears to have specifically targeted the ship’s generator, and the vessel is now at risk of sinking with 30 international human rights activists on board,” the NGO said in a social media post.

The ship issued an SOS distress call after being hit 17 nautical miles (31.5 kilometres) east of Malta, to which Cyprus responded by dispatching a vessel, the charity said.

The coalition is campaigning to end Israel’s blockade of Gaza.