Global leaders and activists have warned the bushfires ravaging Australia are an environmental emergency that must be tackled urgently, as people fleeing apocalyptic-type scenes branded their situation “hell on earth”.

US presidential candidate hopeful Bernie Sanders said the fires, which have killed 18 people and more than 500 million animals, should spur “aggressive” action to tackle the climate crisis.

Hillary Clinton tweeted her support for scientific efforts to repair the natural world, adding: “With Australia on fire and the Arctic in meltdown, it’s clear we’re in a climate emergency.”

And Greta Thunberg shared news and video of the wildfires with the words “this is fine”.

They spoke out as Australia braced itself for a fresh wave of dangerous weather in the next two days, with high winds and temperatures again set to reach 45C or more, threatening to fan the flames that have already devastated the states of New South Wales (NSW) and Victoria.

Tens of thousands of people fled flames approaching seaside towns; ships and helicopters began rescuing thousands more trapped by blazes, and food supplies dwindled after supermarkets closed for safety. At least 17 people are missing.

Prime minister Scott Morrison was heckled by angry residents and told to leave when he visited a town ravaged by fire.

Elsewhere, thick smoke wreaked widespread havoc. Diagnostic tests at Canberra Hospital were cancelled when smoke jammed the MRI scanners; people booked into motels because their houses were clogged with smoke, and face masks sold out.

Drinking water to some towns was cut off; Australia Post was forced to cancel deliveries out of worker safety, and sports events were cancelled.

Mr Sanders warned: “What is happening in Australia today will become increasingly common around the world if we do not aggressively combat climate change and transform our energy system away from fossil fuels. The future of the planet is at stake. We must act.”

Thousands of cars were backed up for hours in small towns south of Nowra, on the southern coast, after fire chiefs ordered a 150-mile stretch to be evacuated.

“It is hell on earth. It is the worst anybody’s ever seen,” said Michelle Roberts, from Mallacoota, on the far southeastern coast, where 4,000 residents and visitors have been stranded on the beach since Monday night.

The HMAS Choules, a naval ship, which arrived off the town on Thursday to save people from the fires engulfing the town, is expected to make two or three voyages over the coming days.

New South Wales declared a state of emergency for the third time in as many months, giving authorities the power to force people to leave their homes. “We want to make sure we are taking every single precaution to be prepared for what could be a horrible day on Saturday,” said state premier Gladys Berejiklian.

Daniel Andrews, premier of neighbouring Victoria, declared a state of disaster for six areas and resorts. Thousands of people have already been rescued from East Gippsland in the state, one of Australia’s largest evacuation operations since a cyclone in 1974.

Five military helicopters are en route to the coast with supplies of water and diesel, the Australian Defence Force said.

Long queues formed outside supermarkets that were still open and petrol stations, while more than 50,000 people were without power.

Najmeh Alyasin, a nurse at Canberra hospital said the building was filled with smoke, “yet we still provide the best service possible”.

At a Sydney hospital, patients reportedly suffered complications from asthma because of the smoke.

Some 18 people have died so far since the fires began in September and following Australia’s hottest December, and more than 200 fires are still burning, threatening several towns.

In NSW, nearly 1,300 homes have been destroyed and another 442 damaged, fire chiefs say. Flames have consumed entire towns and ripped through bushland, with strong winds carrying embers far distances.

Wildlife has been wiped out in unprecedented numbers, raising fears some species will never recover, while the ground has been stripped of insects – vital food for birds and mammals – more deeply than ever. Ecologists estimate more than 480 million animals have been killed, including 8,000 koalas.

Koala Crisis posted: “Not one carer KC has spoken to has seen bees, insects, grubs, worms, snails, beetles, millipedes, for months. Nothing struggles through the dustbowls which are now covering millions of hectares in all states.”

When Mr Morrison visited Cobargo, a historic NSW town where a 63-year-old man and his son died this week, he was met with jeers and shouts that he had “forgotten” about people and was told to “p*** off”.

The prime minister said afterwards he was not surprised people were “feeling very raw”. He said the federal government was sending resources when requested by states, including extra funding and military support but warned that many areas were difficult for emergency workers to safely access.

He blamed a three-year drought and lack of hazard reduction for the crisis, and to criticism of his climate policies, he insisted Australia was meeting the challenge “better than most countries” and fulfilling international targets.

Temperatures are forecast to soar to 45C along the south coast on Friday or Saturday. “It is going to be a very dangerous day,” said NSW rural fire service commissioner Shane Fitzsimmons.

A team of 39 firefighters from the US landed in Melbourne, bringing the number of American and Canadians helping deal with the crisis to almost 100.

Former US presidential candidate Mrs Clinton said she supported the Earthshot Prize, a new effort to inspire problem-solvers to repair the natural world.

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