About 50 firefighters have been called to Foresters Tower in Wood Farm, Oxford, after residents said they could smell smoke.
Ten fire engines from across the city and other parts of the county gathered outside the tower off Wood Farm Road after fire crews were alerted at 12.45pm.
A spokesman for Oxfordshire Fire Service said no blaze has yet been found.
He added: "Residents reported smelling smoke on the fourth floor of the tower and firefighters have been checking the building but so far have not found any fire."
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