BEIRUT — A huge fire broke out Thursday at the Port of Beirut, triggering panic among residents traumatized by last month’s massive explosion there that killed and injured thousands of people.

It was not immediately clear what caused the fire at the facility, which was decimated by the Aug. 4 explosion when nearly 3,000 tons of ammonium nitrate detonated, triggering a shock wave explosion that blasted windows, doors, and walls miles away.