About 12 people are missing and there are two "walking wounded" being treated at the hospital, the British media including The Guardian reported on Saturday
Emergency services are working at the scene Haut du Mont, Pier Road, with surface searches commencing.
Chief of Police Robin Smith said he expected the search and rescue effort to take "a number of days". "It is a pretty devastating scene... the scene is a very dangerous one for all of our staff," he said.
Emergency services said a fire caused by the explosion has been extinguished and have asked people to avoid the area, BBC reported.
A large area of St Helier - including Pier Road, Pier Road car park and South Hill - was cordoned off after the blast just before 04:00 GMT.
The Government of Jersey has asked islanders not to go to the Emergency Department "unless absolutely necessary".
MP/PR