At least 18 contractors working in the area near a chemical leak have complained of respiratory distress, police said, adding that residents in the area had been advised to remain indoors, with their windows shut.
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Cloud and outbreaks of rain will ease away eastwards during the morning, clearing the east coast around midday with sunny spells developing.
Police and emergency services are attending the scene.
Rebel-held east remains at risk while the pumping station supplying the western part of the city is once again in operation.