Democrats in Congress issued subpoenas to compel appearances by four aides to Pompeo as lawmakers probe why President Donald Trump fired the State Department's inspector general, its internal watchdog, in May on Pompeo's advice.

The inspector general, Steve Linick, had been investigating Pompeo's 2019 declaration of an emergency that permitted the Trump administration to sell arms and bypass Congress, where lawmakers have voiced horror at civilian casualties in the Saudi-led offensive in Yemen.

Democratic lawmakers released excerpts from voluntary testimony last week by another figure, Charles Faulkner, who had handled State Department relations with Congress -- and had himself come under scrutiny due to his previous work as an arms lobbyist.

Faulkner told lawmakers that congressional concerns over arms sales to Saudi Arabia were "legitimate" and that State Department staff discussed concerns about civilian casualties, according to excerpts released by the Democrats.

"The administration continues to cover up the real reasons for Mr Linick's firing by stonewalling the committees' investigation and refusing to engage in good faith," said a joint statement by three top Democrats, Representative Eliot Engel, Senator Robert Menendez and Representative Carolyn Maloney.

"Mr Faulkner's testimony depicts a small group of senior State Department officials determined to ignore legitimate humanitarian concerns among their ranks and on Capitol Hill," they said.

The officials subpoenaed include Marik String -- who was promoted to State Department legal advisor on the day of Pompeo's emergency declaration, which had cited tensions with Iran -- and Brian Bulatao, a longtime aide to Pompeo.

Bulatao, the under secretary of state for management, had been due to testify before the House Foreign Affairs Committee in early July, but the State Department requested a delay.

Linick was also said to be investigating allegations that Pompeo and his wife used staff for personal favors such as walking their dog.

Pompeo again defended the firing of Linick last week in an appearance before the Senate Foreign Relations Committee.

Pompeo said that in his previous role as CIA director, he had a "great relationship" with the inspector general, who "took care of the team."

"I know what a good IG can do. Inspector General Linick wasn't that," he said.