SYDNEY (Reuters) - Strong winds stoked more than 100 fires across Australia’s east coast on Tuesday, blanketing Sydney in hazardous smoke and prompting health warnings for the country’s most populous city.

Australia is prone to bushfires in its dry, hot summers, but fierce blazes have been sparked early, in the southern spring, by a long drought and soaring temperatures.

Wildfires have so far this month claimed at least four lives, burnt about 2.5 million acres (1 million hectares) of farmland and bush and destroyed more than 300 homes.

Powerful winds fanned around 130 fires that have been burning across New South Wales and Queensland states for several days, and pushed smoke south to form a thick haze over Sydney, home to around 5 million people.

Officials said the air quality above parts of the harbor city was measured at 10 times hazardous levels on Tuesday and advised people to stay indoors as much as possible as the smoke lingers over coming days.

“We know that heatwaves cause severe illness, hospital admission and even deaths, and that people are more sensitive to heatwaves early in the season,” Richard Broom, director of environmental health at NSW Health said In an emailed statement.

“The combination of heat and poor air quality adds to the risk.”

In NSW, firefighters were scrambling to strengthen fire containment lines ahead of forecast higher temperatures for much of the rest of the week.

“More than 1,300 firefighters are working on these fires, undertaking backburning operations and strengthening containment lines ahead of forecast hot, dry and windy weather, with seven areas under a total fire ban,” the NSW Rural Fire Service said in a statement.

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The current bushfire crisis has mostly been contained to the east coast of NSW and Queensland states, but officials in South Australia warned on Tuesday that forecast near-record temperatures raises the risks in that state.

Australia’s Bureau of Meteorology said temperatures in Adelaide, the capital of South Australia, will hit 42 degrees Celsius (107.6 degrees Fahrenheit) on Wednesday, which coupled with strong winds will create “catastrophic” fire danger conditions.