A ship carrying 16 people and humanitarian aid to Gaza was rocked by explosions early Friday off the coast of Malta, setting the vessel on fire and putting it at risk of sinking, according to the human rights group operating the ship.

The ship and its crew were safe after a tug vessel helped extinguish the blaze following a mayday call, the government of Malta said in a statement.

It did not say what had caused the fire, adding that the authorities were monitoring the ship, which was in international waters in the Mediterranean Sea.

The ship, called the Conscience and operated by a group called the Freedom Flotilla Coalition, had left Tunisia earlier this week carrying human rights activists and aid.

Before going to Gaza, the ship was scheduled to stop in Malta and pick up about 40 more people, including the Swedish activist Greta Thunberg, said Yasemin Acar, a spokeswoman for the group.

It was unclear Friday morning whether the ship, which was damaged, would be allowed to dock in Malta. The source of the explosion has not been determined, and it was not known whether the ship had been deliberately targeted.