SYDNEY, Australia -- Australia's Great Barrier Reef is suffering a "mass bleaching event" as coral comes under heat stress from warmer seas, reef authorities said Friday.

An aerial survey showed coral bleaching at multiple reefs, "confirming a mass bleaching event, the fourth since 2016," said a report by the Reef Authority, which manages the world's largest coral reef system.

Coral suffered despite the cooling effect of the La Nina weather phenomenon which is currently influencing Australia's climate, the authority said.

Scientists say climate change leading to warmer waters is one of the main drivers of coral bleaching.

It occurs when corals expel algae living in their tissues, draining them of their vibrant colours.

Though bleached corals are under stress, they can still recover if conditions become more moderate, the Reef Authority said.

"Weather patterns over the next couple of weeks continue to remain critical in determining the overall extent and severity of coral bleaching across the Marine Park," it said.

The mass bleaching report emerged four days after the United Nations began a monitoring mission to assess whether the World Heritage site is being protected from climate change.

UNESCO's mission is to assess whether the Australian government is doing enough to address threats to the Great Barrier Reef -- including climate change -- before the World Heritage Committee considers listing it as "in danger" in June.

The World Heritage Committee's decision to not list it as such last July surprised many, given UNESCO had recommended the listing just weeks earlier.

When the UN previously threatened to downgrade the reef's World Heritage listing in 2015, Australia created a "Reef 2050" plan and poured billions of dollars into protection.

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Australia declares 'mass bleaching' at Great Barrier Reef