Nearly 30,000 firefighters and support staff were battling blazes across a handful of mainly western US states, officials said Wednesday, after millions of acres and multiple buildings have burned.

A total of 7.16 million acres (2.9 million hectares) have gone up in flames across the country this year, the National Interagency Fire Center said.

That's the earliest the 7 million mark has been reached in the past 20 years, it said, adding that a large portion of this was from Alaskan wildfires several months ago.

The firefighters and other personnel, who totaled 28,884 as of Wednesday, were battling large wildfires in 10 states, center spokeswoman Tina Boehle told AFP.

The northwest quadrant, which is comprised of the states of Washington and Oregon, is the center's "highest priority," she said.

"The last couple of years have been unusual in the Northwest with reduced precipitation. We've been seeing more fires in the Northwest than usual."

A heatwave exacerbating four years of drought is also making California vulnerable.

A total of 41,194 fires were burning across the United States, Boehle said, adding that as of Tuesday, 1,074 structures had been scorched.

"You'll see firefighters from all over assisting with the western wildfires," she said, adding that personnel from the eastern seaboard to Alaska had been tapped.

Canada, meanwhile, has offered up some 100 firefighters plus other personnel and 20 "smokejumpers" who parachute in to fight the blazes.

The center was also in discussion with Australia to provide assistance, she said.

On Tuesday, authorities announced that the US military would be sending troops.

Navy Captain Jeff Davis, a Pentagon spokesman, said 200 troops from Washington state would receive three days' training before heading out to confront the infernos.

Idaho's Soda Fire, which has been one of the largest blazes, was 90 percent contained Wednesday after burning through 283,686 acres.

Amateur video of a "firenado" -- a tornado made of smoke and fire -- that twirled on the sidelines of Soda Fire last week brought the blaze particular notoriety in US media.

Meanwhile since Friday, more than 1,000 firefighters have struggled with a blaze started by lightning in the Chelan, Washington area, where at least 49 buildings have been destroyed and authorities have issued evacuations that affect some 3,000 people, the Seattle Times reported.

The 10 states affected by large fires are Oregon, Washington, Idaho, Montana, California, Nevada, North Carolina, Louisiana, Texas and Arizona, Boehle said.