The company says officials are assessing the situation, but details are now few. Meanwhile, eyewitnesses have taken to social media to post images of black smoke rising from an area inside the refinery.
One resident Bob McVicar reported on Facebook it was a "bed-shaking explosion".
Saint John police responded Monday morning, and they advised residents to avoid the area.
Melanson said he has seen explosions like this on YouTube at oil refineries around the world, but never seen anything like this in Saint John.
The local refinery is a North Atlantic facility and it is still running with no issues, so he doesn't anticipate any disruptions locally.
"As an update on our Saint John refinery incident this morning, we now understand that all employees and contractors working on site have been safely accounted for".
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Irving Oil is a major supplier of oil to the northeastern part of the United States. "We are actively assessing the situation at this time and will share more information when available". "I heard a rumbling, roaring sound".
Black smoke and flames can be seen for miles, higher than the refinery's stacks.
He chose to drive by the refinery and said he was nearby when he felt the vibrations of what he assumes must have been a second explosion.
Saint John resident Doug Bruce, who was around five or six kilometres away from the refinery when it exploded, told Global News "it felt like a truck had run into the building I was in".
Michael Steeves said he was driving about a kilometre away when he saw flames reach as high as 100 feet in the air.
"We're sitting here and watching it burn right now", he said.