The death toll from a powerful blast at a plastics factory in Scotland's largest city Glasgow has risen to seven, firefighters said on Wednesday, adding that they were still hoping to find more survivors. Major Brian Sweeney of the Strathclyde fire brigade said rescue workers still hoped to find people alive in the ruins of the Stockline Plastics building, which was levelled by Tuesday's massive blast. He said knocking noises had been heard from the debris of the four-storey building as dawn broke. Sniffer dogs also gave signs that there might still be people trapped. "This continues to be a rescue operation and I continue to be optimistic," Sweeney told reporters. "If there are people who can be saved, we will save them... We do believe that there are people left in the building." At least 41 people were injured in the blast, 17 of them seriously, emergency services said. Seven of those hurt were pulled from the rubble. The Glasgow Herald newspaper called the blast the worst industrial accident in Scotland in a decade. While ruling out an extremist attack, firefighters have declined to comment on the cause of the explosion, which initial media reports suggested might have been triggered by a gas leak. Stockline Plastics specialised in plastics used to manufacture police equipment, local press reports said. Rescuers toiled overnight at the scene, working under floodlights to pull rubble away from the remains of the building in scenes reminiscent of the aftermath of a major earthquake. They later withdrew to allow specialist rescue dogs to hunt for people trapped in the wreckage alive. Periodically the electricity generator used to power the spotlights was turned off so rescuers could listen for noises. The last person to be rescued was a young woman pulled from the rubble at around 9 pm (2000 GMT) on Tuesday. Sweeney said she was expected to make a full recovery, despite being found in "distressing circumstances". The figure provided by factory managers for the number of people who were inside the building was thought to be unreliable, meaning it was impossible to know how many remained trapped, Sweeney said. The explosion, whcih occurred around midday (1100 GMT) on Tuesday, produced scenes of chaos to the mainly residential area, with scores of bloodied survivors being treated by medics and ferried to hospital. "I was working away as normal. Next thing, there was an explosion," said plant worker Daniel Gilmour, 50, who suffered injuries to his head. "Something came out of a gas oven and hit me on the head," he told the Glasgow Evening Times. "I remember seeing it. The next thing, I woke up and it was pitch black. There was rubble everywhere." "Four of us managed to crawl to a hole in the wall. I don't know how, but we managed to get out. I feel lucky to be alive ... but my worry is for the ones who didn't get out." **PHOTO CAPTION*** Rescue workers survey the damage as others continue to take away debris at a plastics factory in Glasgow. (AFP/Jim Watson)