The category 5 storm, which made landfall early on Friday in Eastern Samar province, has blown towards Boracay island after pummelling the central islands of Samar and Leyte where it toppled power lines, knocked out communications, caused landslides and left streets flooded.
Hundreds of thousands of people have been evacuated and thousands more have fled their homes as Haiyan has torn apart buildings and left whole provinces in the dark and without any telecommunications. Experts are predicting "catastrophic damage" as a result of the super-typhoon.
With speeds at landfall of 195mph and gusts of up to 235mph, Haiyan – the Philippines' 25th typhoon so far this year – is believed to be stronger than the world's last strongest tropical cyclone, hurricane Camille, which was recorded in the US at 190mph in 1969.
Initial reports indicate that two people were killed after being struck by power lines, another by lightning and possibly one more from a falling tree.
The final toll is expected to climb much higher as many of the areas so far damaged are inaccessible to aid or communication, said Mathias Eick of the European commission's humanitarian aid department (Echo), who spoke to the Guardian from Manila.
"It could take a few hours for the authorities to get to villages and communities in the more rural and isolated areas, and there could be such damage that we fear heavy losses," Eick added.
Around 12 airports have been closed – including those in the tourist islands of Palawan and Boracay – and schools and offices shut, with roughly 1 million people in shelters scattered around 29 provinces.
Haiyan's powerful winds could potentially blow off the roofs of storm-proof buildings and suck out their walls due to the sheer force of its energy, experts have said, with an estimated 12 million people at risk.
"There aren't too many buildings constructed that can withstand that kind of wind," said meteorology expert Jeff Masters. "The wind damage should be the most extreme in Philippines history."
The 600km-wide storm – which is called Yolanda in the Philippines – ripped iron roofs off buildings and threw trees across roads, cutting out power to entire provinces, particularly around the storm's eye in Eastern Samar province.
"We've been hearing from my colleagues in [the city of] Tacloban that they've seen galvanised iron sheets flying just like kites," Mai Zamora, of the charity World Vision, told the BBC. "It's actually all around the roads now. The roads are flooded in Tacloban."
President Benigno Aquino III said three cargo planes, 20 navy ships and 32 military planes and helicopters were on standby for rescue operations and to provide relief. "No typhoon can bring Filipinos to their knees if we'll be united," he said in a televised address.
While Haiyan will not hit Mania directly, the capital has been put on low-level alert, with a heavy rain expected around 6pm, said Eick.
"People have been allowed to go home early from their offices and have been stocking up on food, because they're afraid this rain will lead to flooding," he added.
Experts believe most of the damage to come from the typhoon's resulting storm surge.
On Bohol island, which was hit by a 7.3-magnitude earthquake last month that killed 200 people, residents have been waiting out the typhoon in the dark, without power or water supplies, said Jackie Pinat of Catholic aid agency Cafod.
"Many people on the island lost their homes in the earthquake, and many structures are unsafe," she said. "Most people in the coastal villages around Maribojoc are still in designated evacuation centres."
Typhoon Bopha in 2013 destroyed much of the southern islands, killing about 1,100 people and causing over $1bn worth of damage.
