San Francisco city attorney Dennis Herrera said Monday that his office may be forced to take court action over President Trump Donald John TrumpBiden on Trump's refusal to commit to peaceful transfer of power: 'What country are we in?' Romney: 'Unthinkable and unacceptable' to not commit to peaceful transition of power Two Louisville police officers shot amid Breonna Taylor grand jury protests MORE's appointment of Matthew Whitaker as acting attorney general.

“I have serious questions about the legality of Whitaker serving as the Attorney General of the United States," Herrera said in a statement. "At no point in his Justice Department tenure has Mr. Whitaker been confirmed by the Senate. That appears to be unprecedented for someone serving as the country’s top law enforcement officer."

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Herrera later added that "we need to make sure that the president’s appointment here is lawful.”

His comments come about a week after Attorney General Jeff Sessions Jefferson (Jeff) Beauregard SessionsGOP set to release controversial Biden report Trump's policies on refugees are as simple as ABCs Ocasio-Cortez, Velázquez call for convention to decide Puerto Rico status MORE resigned at Trump's request. Trump announced on Twitter that Whitaker, Sessions's former chief of staff, would assume his boss's position.

In a letter to the Department of Justice, Herrera voiced concerns over the fact that San Francisco has four cases going forward in court in which Sessions is named the defendant.

One of those cases involves the legality of Trump's executive order targeting "sanctuary cities."

“Given that Attorney General Sessions’ resignation will affect San Francisco’s pending litigation, I ask that you provide legal justification for the appointment of Acting Attorney General Whitaker," Herrera said in his letter, according to the city attorney's website. "Otherwise, San Francisco may be forced to move the court to obtain additional guidance.”

The Department of Justice declined to comment to The Hill.

The Trump administration has faced growing scrutiny in recent days over its decision to name Whitaker as acting attorney general.

Whitaker, who has been publicly critical of certain elements of special counsel Robert Mueller Robert (Bob) MuellerCNN's Toobin warns McCabe is in 'perilous condition' with emboldened Trump CNN anchor rips Trump over Stone while evoking Clinton-Lynch tarmac meeting The Hill's 12:30 Report: New Hampshire fallout MORE's investigation, is now tasked with overseeing it.

Senate Minority Leader Charles Schumer (D-N.Y.), House Minority Leader Nancy Pelosi (D-Calif.) and other Democrats have called for Whitaker to recuse himself from overseeing the Russia probe.

-Updated 4:15 p.m.