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.

Devastating wildfires rage across Australia: In pictures Show all 40 1 /40 Devastating wildfires rage across Australia: In pictures Devastating wildfires rage across Australia: In pictures A firefighter hosing down trees and flying embers in an effort to secure nearby houses from bushfires near the town of Nowra in the state of New South Wales on 31 December 2019 AFP via Getty Devastating wildfires rage across Australia: In pictures Smoke billows from a huge bushfire that has torched over 200,000 acres of land in East Gipplsand, Victoria on 2 January EPA Devastating wildfires rage across Australia: In pictures Residents look on as flames tear through bushland in Lake Tabouriee, Australia on 4 January on 4 January Getty Devastating wildfires rage across Australia: In pictures Boats are pulled ashore as smoke and wildfires rage behind Lake Conjola on 2 January Robert Oerlemans via AP Devastating wildfires rage across Australia: In pictures A firefighting helicopter tackles a bushfire in East Gippsland, Victoria on 31 December EPA Devastating wildfires rage across Australia: In pictures A firefighter gives water to a parched koala in Cudlee Creek, South Australia AP Devastating wildfires rage across Australia: In pictures Firefighters tackle a blaze as it tears through a farm in New South Wales on 21 December AP Devastating wildfires rage across Australia: In pictures The sky is turned red over East Gippsland as fires continue to rage through Australian bushland on 4 January Getty Devastating wildfires rage across Australia: In pictures A kangaroo near bushfires in Nowra AFP/Getty Devastating wildfires rage across Australia: In pictures An aerial view of a bushfire near Bairnsdale State Government of Victoria/EPA Devastating wildfires rage across Australia: In pictures Firefighters work to tackle a blaze on the outskirts of Sydney on 31 December 2019 Getty Devastating wildfires rage across Australia: In pictures A firefighting helicopter dumps water on a bushfire on the outskirts of the town of Bargo near Sydney Getty Devastating wildfires rage across Australia: In pictures Children play at the showgrounds in the southern New South Wales town of Bega where they are camping after being evacuated from nearby sites affected by bushfires AFP via Getty Devastating wildfires rage across Australia: In pictures A satellite image of the Batemans Bay showing smoke and fire from wild bushfires European Union, Copernicus Sentinel Data via REUTERS Devastating wildfires rage across Australia: In pictures The afternoon sky glows red from bushfires in Nowra AFP/Getty Devastating wildfires rage across Australia: In pictures Burning embers cover the ground as firefighters battle against bushfires around the town of Nowra AFP via Getty Devastating wildfires rage across Australia: In pictures The sky glows red as bushfires continue to rage in Mallacoota, Victoria Jonty Smith via Reuters Devastating wildfires rage across Australia: In pictures The remains of burnt out buildings along a main street in the New South Wales town of Cobargo AFP/Getty Devastating wildfires rage across Australia: In pictures Firefighters try to protect homes around Charmhaven, New South Wales NSW Rural Fire Service/AP Devastating wildfires rage across Australia: In pictures Wildfires rage under plumes of smoke in Bairnsdale Glen Morey via AP Devastating wildfires rage across Australia: In pictures Business owners stand in front of their shop which was destroyed by a bushfire in Cobargo EPA Devastating wildfires rage across Australia: In pictures A helicopter dumping water on a fire in Victoria's East Gippsland region Department of Environment, Land, Water and Planning/AFP Devastating wildfires rage across Australia: In pictures Nowra AFP via Getty Devastating wildfires rage across Australia: In pictures Think smoke from bushfires fills the air in eastern Gippsland Getty Devastating wildfires rage across Australia: In pictures "Carmelised" snow caused by dust from Australian bushfires is seen near Franz Josef glacier in the Westland Tai Poutini National Park, New Zealand Reuters Devastating wildfires rage across Australia: In pictures Firefighters hose down trees as they battle against bushfires around the town of Nowra AFP via Getty Devastating wildfires rage across Australia: In pictures Smoke billowing from a fire burning at East Gippsland, Victoria. More than 800,000 hectares have been burnt in East Gippsland EPA/DELWP Gippsland Devastating wildfires rage across Australia: In pictures Smoke billowing from a fire burning at East Gippsland EPA/DELWP Gippsland Devastating wildfires rage across Australia: In pictures Fire clouds are formed over the mountains' range near Bredbo, New South Wale, Australia December December 31, 2019. Picture taken December 31, 2019. LIFES.A.BREEZE via REUTERS THIS IMAGE HAS BEEN SUPPLIED BY A THIRD PARTY. MANDATORY CREDIT. NO RESALES. NO ARCHIVES. LIFES.A.BREEZE LIFES.A.BREEZE via Reuters Devastating wildfires rage across Australia: In pictures Smoke and wildfire rage behind Lake Conjola Robert Oerlemans via AP Devastating wildfires rage across Australia: In pictures A house and van are seen destroyed after bushfires ravaged the town of Bilpin, west of Sydney AFP via Getty Devastating wildfires rage across Australia: In pictures A helicopter fighting a bushfire near Bairnsdale in Victoria's East Gippsland region State Government of Victoria/AFP Devastating wildfires rage across Australia: In pictures Fire and Rescue personal run to move their truck as a bushfire burns next to a major road and homes on the outskirts of the town of Bilpin Getty Devastating wildfires rage across Australia: In pictures Amy, left, and Ben Spencer sit at the showgrounds in the southern New South Wales town of Bega where they are camping after being evacuated from nearby sites affected by bushfires AFP via Getty Devastating wildfires rage across Australia: In pictures A firefighter sprays foam retardant on a back burn ahead of a fire front in the New South Wales town of Jerrawangala AFP via Getty Devastating wildfires rage across Australia: In pictures Two bushfires approach a home located on the outskirts of the town of Bargo Getty Devastating wildfires rage across Australia: In pictures Property damaged by the East Gippsland fires in Sarsfield, Victoria EPA Devastating wildfires rage across Australia: In pictures Nowra AFP via Getty Devastating wildfires rage across Australia: In pictures Property under threat from the East Gippsland fires in Sarsfield EPA Devastating wildfires rage across Australia: In pictures The main street of the New South Wales town of Bombala is pictured shrouded in smoke from nearby bushfires AFP via Getty

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.

Cars line up to leave the town of Batemans Bay (AFP/Getty)

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.

A kangaroo near bushfires in Nowra (AFP/Getty)

“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.

Property under threat from the East Gippsland fires in Victoria (EPA)

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.

Firefighters at work in New South Wales (AFP)

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.

ABC photographer Matt Roberts in shock at seeing his sister’s house destroyed (EPA)

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.”

Daniel Andrews, the Victoria premier, comforts a couple who lost their home (EPA)

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.

Dog walkers shrouded in smoke near the Batemans Bay bridge (AFP ) (AFP)

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.