A group of wildfires blazing in the western US state of Washington have become the largest in the state's history, and 2015 has become the state's worst fire year on record, officials said Monday.

The hard-hit state is only one of many in the region suffering from what has become one of the worst fire seasons in recent history for the United States as a whole.

The so-called Okanogan complex of wildfires in the north-central part of Washington state has already surpassed last year's record-breaking Carlton fires, US Forest Service fire spokesman Mike Ferris told AFP.

The Okanogan fire, which is still only 10 percent contained, had grown to more than 250,000 acres (104,000 hectares) Monday, according to the National Interagency Fire Center (NIFC).

Three firefighters were killed battling a portion of the Okanogan fire last week and a fourth remained in critical condition. Currently, 1,250 people are helping quell the blaze.

Statewide, 19 wildfires greater than 1,000 acres in area are currently raging, according to the NIFC.

The active fires have ravaged more than 630,000 acres, destroying approximately 200 homes and threatening another 12,000, according to Washington state officials.

That combined damage makes 2015 "certainly the most severe in terms of timing of multiple large fires simultaneously," Jaime Smith, a spokeswoman for state Governor Jay Inslee, told AFP Monday.

"Last year, up until this point considered the worst wildfire season in the state, we saw about 419,000 acres burn."

Smith said more than 1,200 people had been evacuated due to the fires and that the Red Cross was assisting with temporary emergency housing.

President Barack Obama has approved Inslee's request for a federal declaration of emergency, which releases federal funds to help the state pay for the cost of fighting the blazes.

The Okanogan fire alone has already cost $8.5 million to date, the NIFC said Monday.

Seven hundred members of the Washington National Guard were added to the state-wide firefighting effort Sunday, and helicopters from surrounding states were brought in to help.

Two hundred soldiers from the US Army who had been stationed at Joint Base Lewis-McChord in western Washington were also deployed to assist the firefighting effort.

A white haze blanketed the northwestern United States in satellite images released by NASA Monday, obscuring the green forests of the Rocky and Cascade mountain ranges.

Twenty active fires more than 1,000 acres in area are currently raging in California alone. Another 14 greater than 1,000 acres remain active in Oregon.

Nationwide, nearly 7.5 million acres have burned already this year -- almost three times the acreage that had burned at this point in 2014.

Currently, more than 27,000 firefighters and support personnel are battling major blazes across the country, NIFC said Monday.

Fighting those fires cost federal agencies $1.5 billion last year -- not counting the expenditures by state and local governments or the losses caused by the fires.