Irving Oil released a tweet saying 'We can confirm that a major incident has occurred at our Saint John refinery this morning'.
We will update as more information becomes available.
Tom Kloza, global head of energy analysis at Oil Price Information Service told CNBC 'We probably import more gasoline from that single refinery than any other single refinery around the world, ' he said.
Scott said the unit was quickly shut down after the blast.
Michael Steeves, who lives in Saint John, said he was driving about a kilometre away when he noticed what was happening at the refinery.
Irving's Saint John refinery is the largest in Canada.
A local hospital had issued an "orange alert", meaning it was preparing for a possible influx of patients, while the City of Saint John advised residents to shelter in place.
The employees said the explosion happened in an active part of the refinery, and not in a section now undergoing a maintenance shutdown.
"We're sitting here and watching it burn right now", he said.
They looked outside the window and saw black smoke and flames exceeding the height of the stacks at the refinery. It produces 300,000 barrels a day of gasoline, heating oil and jet fuel, exporting half of it to the Northeast through terminals that include Searsport, Portland and Portsmouth, New Hampshire.
"I heard a rumbling, roaring sound", Guimond said.
He chose to drive by the refinery and said he was nearby when he felt the vibrations of what he assumed must have been a second explosion.
Saint John police are responding as are fire officials.
When he drover down the corner he saw flames shooting into the sky about 100-feet high.
"The following streets are closed: Loch Lomond at Commerce, Grandview at Champlain, Grandview at Bayside, and Loch Lomond at Macdonald". He said the incident reminded him of a similar event in the late 90s, when the same refinery suffered a similar explosion which left one person dead.
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- Oil refinery explosion in Canada shakes city of Saint John