The Government Accountability Office is reviewing whether the Trump administration violated any congressional appropriations laws by failing to formally inform Congress about its decision to place a hold on nearly $400 million in security aid to Ukraine earlier this year.

During a Senate hearing last week, Sen. Chris Van Hollen, D-Md., asked Gene Dadaro, the U.S. comptroller general, whether any rules had been broken by the White House.

"If you could just get back to me on whether you agree, and number two, what's our recourse as a Congress if they don't do it," Van Hollen asked Dadaro.

"We are conducting the review requested by Sen. Van Hollen," GAO spokesman Charles Young confirmed to CNBC on Thursday. "There is no time frame for completion yet." Young emphasized that Van Hollen's request was for a legal opinion and not for an investigation.

Nonetheless, the decision under review, namely, to freeze aid to Ukraine, is at the center of the House impeachment inquiry, making it difficult to separate the review from the deep partisan divisions underlying the impeachment probe itself.

That inquiry is examining whether military aid to Ukraine was conditioned on Ukraine agreeing to open probes involving Trump's political opponents. These include Trump's requests that President Volodymyr Zelensky "look into" unsubstantiated allegations against former Vice President Joe Biden and his son Hunter, who served on the board of a Ukrainian company, and that Zelensky launch a probe into a discredited theory that Ukraine interfered in the 2016 presidential election.

Biden is currently one of the front-runners in the 2020 Democratic presidential primary, and he could eventually face Trump in the general election.

Trump has defended the nature of his July phone call with Zelensky, saying that he was focused on limiting corruption in Ukraine, not "a political campaign against the Bidens."

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However, the Defense Department told Congress in May that Ukraine had taken "substantial actions to make defense institutional reforms for the purposes of decreasing corruption (and) increasing accountability."

Under federal law, presidents have limited ability to withhold congressionally allocated funds. The aid was released in mid-September after bipartisan pressure from lawmakers. A whistleblower complaint about the controversial July phone call between Trump and Zelensky released in late September led to the impeachment proceedings currently underway.

Bill Taylor, acting ambassador to Ukraine, testified to lawmakers during an impeachment hearing in October that EU Ambassador Gordon Sondland told him the president was withholding military aid to Ukraine until it publicly pledged to investigate the Bidens.

Sondland confirmed Taylor's account on Tuesday, when he returned to Congress to amend his original October testimony. Sondland said testimony from other officials "refreshed his recollection."