LOS ANGELES, California — Two firefighters battling a wildfire in the United States have been arrested by border patrol agents, immigration officials said Thursday, as President Donald Trump's migration crackdown gathers pace.

Federal agents held crews from private fire contractors for several hours on Wednesday as they readied to help tackle the 9,000-acre (3,600-hectare) Bear Gulch Fire in Washington state, reports said.

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Agents lined the 44 people up and ordered them to produce identification, firefighters told the Seattle Times, with crew members told they were not allowed to film the encounter.

"You risked your life out here to save the community," one firefighter, who was not named, told the paper. "This is how they treat us."

US Customs and Border Protection (CBP) said Border Patrol officers were there at the request of rangers who wanted to verify that the names provided on contractor rosters were accurate.

"Several discrepancies were identified, and two individuals were found to be present in the United States illegally, one with a previous order of removal," a CBP statement said Thursday.

Those two people were arrested and detained, the CBP said, while the remaining 42 were removed from federal lands in what the agency characterised as a "contract termination and enforcement" following a criminal investigation.

The operation "did not interfere with firefighting operations or the response to any active fires in the area, nor did it pose any danger to the surrounding community."

Wednesday's immigration action was unusual in that such operations have customarily not been carried out around natural disasters or sites of emergency.

Firefighters arrested by US immigration officials

But under an election campaign pledge to undertake the largest deportation in US history, Trump's administration has continually pushed the envelope, carrying out widespread arrests -- often in Democratic areas -- that have provoked anger.

Washington's Democratic Governor Bob Ferguson said he was seeking details about the operation.

"Deeply concerned about this situation with two individuals helping to fight fires in Washington state," he wrote on social media.

"I've directed my team to get more information about what happened."

The Bear Gulch Fire erupted in July, caused by some kind of human activity, official information shows.

It was just 13 percent contained and characterized as "creeping" through mature conifer woodland.