A US federal prosecutor has resigned after reportedly facing pressure over investigations into two of Donald Trump's political foes and hours after the president called for his exit.

Erik Siebert, US attorney for the Eastern District of Virginia, told staff of his resignation via an email, the New York Times and other US media outlets reported.

The move came just hours after Trump told reporters in the Oval Office "I want him out" in response to a question about firing him.

Siebert had been under pressure to prosecute two of Trump's political adversaries, former FBI director James Comey – whom Trump fired in 2017 – and New York Attorney General Leticia James, the Times reported, saying that some officials in the administration had argued for him to keep his position.

The federal prosecutor had recently told Justice Department leadership he was declining to prosecute Comey over allegations he lied to Congress, and that there was insufficient evidence to charge James with mortgage fraud, officials familiar with the matter told the Washington Post.

Comey was fired while leading a probe into whether any members of the Trump campaign had colluded with Moscow to sway the 2016 presidential vote, and has been a vocal critic of the Republican president.

James, as New York's state prosecutor, brought a US$464 million case against Trump, alleging he and his company had unlawfully inflated his wealth and manipulated the value of properties to obtain favorable bank loans or insurance terms.

She, like several other Democratic officials, has been accused by a close Trump ally, Federal Housing Finance Agency director Bill Pulte, of falsifying documents on mortgage applications.

Asked about the case against James on Friday, Trump said: "It looks to me like she's really guilty of something, but I really don't know."

Siebert, a graduate of Virginia Military Institute and a former Washington police officer, led a team of approximately 300 prosecutors in a jurisdiction that often handles major cases related to national security. (AFP)