The cause of the blast, which occurred on Monday evening, was not yet known, said Minister for Energy and Mining Shakib Khalil.
Khalil, who visited the site, said three liquefaction units had been destroyed.
The plant, located about 500km east of the capital Algiers, is part of a huge complex which liquefies Algerian gas from the Sahara, much of it for export to Europe.
The director of the plant said the blast occurred at 17:40 GMT on Monday, in an area where many people were working.
Windows were blown out in nearby houses and the resulting fire was still raging in the early hours of Tuesday morning.
It was not immediately known how many people were working in the area at the time of the blast, which meant that the final toll could be higher.
The local official, in charge of health in the Skikda region, told state radio the fire had been brought under control early on Tuesday.
Pictures screened on Algerian television showed scenes of devastation.
Complex not safe
Some of the injured were taken to a hospital at Annaba, another port 600km east of Algiers, and others could be taken to the Ain Nadja military hospital specialising in burns, in the suburbs of Algiers.
Algerian state television said late on Monday there was no danger of a new explosion, but the complex was not safe. It said the blast had not caused any environmental damage as natural gas is not toxic.
A crisis team was formed by the Algerian state oil and gas company, Sonatrach, to deal with the accident.
The coastal Skikda complex includes six plants for the processing of gas and oil products and employs 12,000 people. It normally exports 15 million tonnes of liquefied natural gas and oil products to Europe each year.
Algerian state television said there was no danger of a new explosion, but the complex was not safe. The explosion has not caused any environmental damage as natural gas is not toxic.