US President Donald Trump has defended his embattled head of the Environmental Protection Agency, Scott Pruitt, saying the official’s travel and security expenses were reasonable.

Taxpayers have paid more to protect Pruitt than past EPA chiefs but those expenses are justified since Pruitt has been under threat, Trump wrote.

“Scott Pruitt has received death threats because of his bold actions at EPA,” Trump wrote on Saturday.

The costs of Pruitt’s security detail have come under scrutiny recently as have the costs of his travel and housing in Washington. Trump defended those costs, too.

“Rent was about market rate, travel expenses OK. Scott is doing a great job!” Trump wrote.

Activists protest EPA chief Scott Pruitt’s policies in Washington. Photo: AP/Andrew Harnik

The head of a congressional oversight panel is investigating Pruitt’s use of a condominium tied to an energy lobbyist, an aide said on Saturday.

The House of Representatives Oversight Committee, chaired by Republican Representative Trey Gowdy, has begun looking into Pruitt’s housing arrangements, according to the aide, who was not authorised to speak publicly.

The EPA had no immediate comment. An agency spokesman told the Associated Press that Pruitt has faced “unprecedented” threats and keeping the official safe has been costly.

Posters decrying Environment Secretary Scott Pruitt’s sweetheart rent deal are plastered all over Washington. Photo: AP/Carolyn Kaster

Gowdy’s probe adds to the pressure on Pruitt, a vocal critic of mainstream climate change science who sued the EPA more than a dozen times when he was Oklahoma’s attorney general.

Many Democrats and even a few Republicans have said he should resign or be fired.

Lawmakers have been scrutinising Pruitt for renting a room in a high-end townhouse co-owned by the wife of energy industry lobbyist Steven Hart for $US50 a night, an arrangement that has drawn fire from ethics experts.

Media reports say the payment is less than one-third the price of similar properties.

Hart lobbies for companies regulated by the EPA.

Pruitt is also under fire for frequent first-class air travel and for purchasing costly items for his office, including a soundproof telephone booth.