US President Barack Obama declared a state of emergency in Washington state on Friday as at least 70 huge wildfires continued to rage there and at least four other Western states—Oregon, Idaho, California and Montana—threatening more populated areas amid the continuing heat wave.

More than 30,000 firefighters and support personnel have been battling the Western inferno that has already claimed the lives of at least 13 firemen this year, including the three who died fighting the Washington fire on Wednesday night, officials said.

The forest fires this year have already scorched 11,400 square miles (29,500 sq km) of brush and dry timber in an area roughly equivalent to the land mass of Vermont and Delaware combined, officials said.

With manpower from the US Forest Service already stretched to its limit, the federal government has called on the US Army and the National Guard to help in containing the fires. Firefighters from Canada, Australia and New Zealand have also joined in the effort.

The fires in Washington state have forced the evacuation of residents in at least two towns as flames closed in.

The state of emergency declared by President Obama authorises the US Department of Homeland Security and the Federal Emergency Management Agency to coordinate relief efforts in 11 counties and several Indian reservations in Washington state that were hit hardest by the wildfires.

The US Forest Service has sent prayers to the families of three firefighters killed while battling the fast-growing wildfire in Washington state.

Mike Williams, Forest Supervisor on the Okanogan-Wenatchee National Forest, said the agency is still reeling from the loss of the firefighters' lives.

"We are devastated by the tragic loss of three of our Forest Service firefighters," Williams said.

He added that the firefighters' families are already being notified about the tragedy.

"We are working with county and state partners to notify the families of those lost. Our hearts and prayers go out to the families and fellow crew members of these brave firefighters," Williams said.

The three firefighters were later identified as as Tom Zbyszewski, 20, Andrew Zajac, 26, and Richard Wheeler, 31.

The three men, who all belonged to an engine crew from Okanogan-Wenatchee National Forest in central Washington, lost their lives after the vehicle they were riding crash and was consumed by the blazing fire.

Washington Gov. Jay Inslee described the three firefighters as "big heroes protecting small towns."

"We know that these fires have burned a big hole in our state's heart," Inslee said.

He also urged all Washington residents to "thank a firefighter."

The Secretary for the United States Department of Agriculture, Tom Vilsack, also extended condolences to the bereaved families of the firefighters.

"We mourn the loss of the brave firefighters whose commitment to duty was so deep that they gave their own lives to protect others. We also extend our profound sympathies to the families and loved ones of the fallen, whose sacrifices are equally worthy of honour, and our thoughts and prayers are with those who are recovering from injuries sustained in the line of duty," Vilsack said in a statement.