Thousands of Australian firefighters are battling 70 bushfires threatening suburban homes in Sydney, which is blanketed by huge plumes of smoke.

More than 20 houses have already been destroyed as conditions deteriorate rapidly, with high winds fanning the flames tearing through the bone-dry bush land.

There is no doubt the fires are stirring and threatening and we can expect there is going to be a lot more activity before the day is out

State Emergency Services Minister Bob Debus

Helicopters carrying thousands of litres of water are helping efforts to douse the flames and the authorities have offered to send in the armed forces to tackle the fires, most of which erupted on Wednesday.

Although fires are natural to the arid bush in Australia - which is suffering one of its worst droughts in 100 years - senior officials believe some were started deliberately.

A police spokesman said a young student had been charged with lighting one of the fires.

Roads have been closed, and train services and power supplies disrupted in the city of four million.

Residents have been evacuated from the areas most in danger and some schools have been shut.

The BBC's Dominic Hughes in Sydney says the speed and ferocity of the bush fires now burning in northern and southern suburbs has shaken even the most experienced of the 3,000 firefighters trying to save homes.

'Blackened rubble'

New South Wales (NSW) Emergency Services Minister Bob Debus said that after a relatively calm morning, fires started to pick up in the early afternoon.

"There is no doubt the fires are stirring and threatening and we can expect... that there is going to be a lot more activity before the day is out," he was quoted as saying.

Preventive measures are being taken

A 73-year old man died in a far northern suburb of the city - possibly from a heart attack, as he tried to herd his horses to safety and save his home, the state's rural fire services commissioner, Phil Koperberg, said.

Fifteen houses had been burnt to mounds of blackened rubble in the north-western suburb of Glenorie - one of the worst affected areas on Wednesday - and on Thursday some residents were hosing down buildings while others packed and fled, Reuters news agency reported.

One terrified kangaroo had sought refuge in a swimming pool, it said.

Dozens of water bombing helicopters and planes are involved in the frantic efforts to put out the fires or stop them spreading.

Worst fire season?

On Wednesday fire officials said at least 30 fires broke out within an hour of each other, taking everybody by surprise.

In the intense heat, flames leapt 60 metres (200 ft) and oil-filled eucalyptus trees exploded, the Associated Press news agency reported.

Firefighters take water wherever they can

Many of the fires are thought to have been started through negligence - by cigarettes discarded from car windows, but there is also a suspicion that some were lit deliberately.

"This is the most extreme form of anti-social behaviour you can see in our society," the state premier Bob Carr said Thursday after touring the devastated region.

December and January tend to be the worst months for bush fires, and firefighters fear this is the beginning of one of the most dangerous seasons they have ever faced.

Last Christmas, a wall of flames ringed Sydney, burning through 777,000 ha (1.9 million acres) and destroying nearly 200 homes.