Most of the dead, including women and children, were from the same family, the source at the Harish hospital said, adding that three of five people wounded were in a critical condition after Monday evening's strike.
It was not immediately clear who carried out the raid.
The attack has not been claimed by forces loyal to strongman Khalifa Haftar, which have been besieging Derna for months and regularly conduct air strikes against the eastern city.
A spokesman for Haftar's forces, General Ahmed al-Mesmari, on Tuesday condemned what he called "a terrorist act", and said that "no plane carried out an air raid in this zone" at the time the attack took place.
The United Nations mission in Libya condemned the attack and the Tripoli-based Government of National Accord said it "will identify as soon as possible the origin of the raids... and take the necessary measures".
Controversial strongman Haftar supports an administration based in the country's east that has refused to recognise the GNA.
The GNA added that it had written to the UN Security Council asking that it investigate this "a war crime".
Like the UN, the GNA has called for the siege of Derna to be lifted to allow in humanitarian aid.
Derna, some 900 kilometres (560 miles) east of Tripoli, was known as a jihadist bastion even before the 2011 uprising that toppled and killed long-time dictator Moamer Kadhafi.
Haftar's forces are waging a campaign against jihadist groups, including efforts to capture the city from the "Revolutionary Shura Council of Derna", a coalition of militias close to Al-Qaeda.
Jihadists in the conflict-torn North African country have frequently accused Haftar's main international backers, Egypt and the United Arab Emirates, of participating in deadly raids.
Egypt in late May said it launched strikes in the Derna area after a bloody attack on its soil claimed by the Islamic State group killed 29 people.