A decision will be made today on the deployment of Australian firefighters to the United States to help battle bushfires across the north-west of the nation.

Key points: The US has asked for 188 specialist firefighters from Australia and New Zealand

The US has asked for 188 specialist firefighters from Australia and New Zealand They will be deployed for up to 42 days in the California area

They will be deployed for up to 42 days in the California area Fires across north-west America have burned more than 3,760 square kilometres over the northern summer

US firefighters have worked to put out 89 large fires, which have burned more than 3,760 square kilometres over the peak summer period.

The National Multi-Agency Coordination Group (MAC) in the US formally requested assistance over the weekend, seeking 188 specialist firefighters from Australia and New Zealand.

The firefighters would be deployed for up to 42 days in the California area.

Candidates from each state will be placed in a national pool, with the most suitable candidates chosen this afternoon ahead of their deployment to the US on Friday.

Homes levelled by the Carr Fire line the Lake Keswick Estates area of Redding. ( AP: Noah Berger )

Forest bushfire experience important

South Australian Country Fire Service spokesman Brett Williamson said the most suitable firefighters to come forward in South Australia so far had experience fighting fires in areas with rough terrain and forests.

"We are finding it is more the state-based ones — in particular [those from] the South East and the Port Lincoln area … that are the ones who will probably have the beneficial skills that the United States [is] searching for," Mr Williamson said.

"In particular, they are after people that have had experience in forestry fighting in especially tough terrain."

The US has asked for paid staff rather than volunteers.

"At this stage, we are still calculating the numbers of who is available, who will be available from their employers to be released and then they will go into a national pool and that national pool will then basically be cherry-picked for the best people that are suitable for the job," Mr Williamson said.

Mr Williamson said the firefighters had a lot to do before departing for the US on Friday and could expect an extremely challenging scenario once they had boots on the ground in the fire zones.

A historic schoolhouse went up in flames in Shasta. ( AP: Noah Berger )

"It's not unusual in a large bushfire situation, or wildfire as they call them, to ask for international aid, but the ones that we are seeing in the north-western parts of the United States are pretty bad," he said.

"They are currently battling 89 large fires and they have had losses of more than 376,000 hectares in 14 states across the north-western parts of the United States."

A firefighter hoses down flames as a wildfire advances onto a residential district. ( AP: Marcio Jose Sanchez )

He said the Australian and New Zealand firefighters would be needed primarily to "replenish the ranks" and provide some relief to local firefighters who have been working non-stop to get the bushfires under control.

"So you can imagine its a fairly stressful time for them there," Mr Williamson said.

"Fires don't stop for sleep so this is a 24-hour battle for the American firefighting forces and extra people with the specialised skills that they need will help a lot."

Sorry, this video has expired 'It's burning differently it's burning more aggressive': California firefighters struggle to contain wildfire

50,000 people evacuated

More than 50,000 people have been evacuated in the northern California wildfires, which have killed six people.

On Sunday, local officials estimated about 12,000 firefighters were battling significant fires in the US state, with considerably greater fire activity in California this year than at the same time last year, with the worst of the fire season yet to come.

Fire officials have said the behaviour of the Carr fire, the most deadly and destructive fire burning at the moment, was erratic and had burned more than 360 square kilometres of drought-parched vegetation since last Monday.

This included the destruction of more than 800 homes.

Around 3,500 firefighting personnel and a squadron of 17 water-dropping helicopters were working to carve buffer lines around the fire's perimeter.

On Saturday, US President Donald Trump declared the fire an emergency, authorising federal funds for disaster relief efforts.