It is considered the single most deadly bombing raid in history.

Seventy years ago today, US forces firebombed Tokyo to force the Japanese to an early surrender in the dying months of World War II.

The atomic bombs of Hiroshima and Nagasaki have dominated the retelling of WWII history, but as a single attack the bombing of Tokyo was more destructive.

The firestorms killed about 100,000 civilians and wiped out about half of the city.

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The US military had waited for a clear and windy night to inflict maximum damage, and on March 9, 1945 the conditions were perfect.

Three hundred B29 bombers dropped nearly 500,000 cylinders of napalm and petroleum jelly on the most densely populated areas of Tokyo.

The raid, which came a month after the firebombing of Dresden, brought mass incineration of civilians to a new horrific level.

Kisako Motoki, then 10 years old, fled to a bridge to seek refuge after her parents and brother had just been burnt to death.

The firestorm, hundreds of metres high and fuelled by strong winds, quickly turned 40 square kilometres of Tokyo into an inferno.

"I saw melted burnt bodies piled up on top of each other as high as a house," Ms Motoki said.

"I saw black pieces, bits of bodies everywhere on the ground and burnt corpses in the water.

"I couldn't believe this was happening in this world."

Survivor says US should be held to account

The firebombing of Tokyo was designed to terrorise and bomb the Japanese into surrender.

It was also seen as payback for the Pearl Harbour attacks and the mistreatment of Allied prisoners of war.

In just two days, more than 100,000 people were killed, a million were maimed and another million were made homeless.

Ms Motoki said she could never forget.

"At the time, my mind went blank and I was stupefied in shock," she said.

"Now 70 years have passed, but those scenes of bodies can't leave my mind.

"It was worse than hell."

A photograph taken from one of Tokyo's tall buildings shows an area flattened by bombs. ( Australian War Memorial )

Now close allies, the US and Japan have mostly forgotten the Tokyo firebombing, but another survivor, Haruyo Nihei, said it was important the children of today remembered.

She holds regular seminars for school children at a privately funded museum dedicated to the victims.

"It's likely Japan will be involved in a future war, so I want our children to understand war destroys everything — families, buildings and culture," she said.

Ms Nihei also wanted the Japanese and US governments to acknowledge and apologise for the firebombings.

She said American claims that the bombings targeted factories were false.

"There were no big military factories in the areas they bombed on March 9. They did it as punishment," Ms Nihei said.

"I believe they should be held accountable for war crimes too."

US Air Force general Curtis LeMay, the man who ordered the raids across Japan, once said the US military "scorched and boiled and baked to death more people in Tokyo on that night ... than Hiroshima and Nagasaki combined".

He acknowledged that if he had been on the losing side, he would be charged with war crimes.

And the evidence lies deep in the vaults of a memorial in central Tokyo, where large urns contain the ashes of more than 100,000 civilians.

Most remain unidentified, but what is known is that the vast majority were women, children and elderly — the men were on the frontlines.