WASHINGTON -- A federal appeals court in Washington declined to order the dismissal of the Michael Flynn prosecution, permitting a judge to scrutinize the Justice Department's request to dismiss its case against U.S. President Donald Trump's former national security adviser.

Monday's decision keeps the case at least temporarily alive and rebuffs efforts by both Flynn's lawyers and the Justice Department to force the prosecution to be dropped without further inquiry from the judge, who has for months declined to dismiss it. The ruling is the latest development in a criminal case that has taken unusual twists and turns over the last year and prompted a separation of powers tussle involving a veteran federal judge and the Trump administration.

The Flynn conflict arose in May when the Justice Department moved to dismiss the prosecution despite Flynn's own guilty plea to lying to the FBI about his contacts with the Russian ambassador during the presidential transition period.

But U.S. District Judge Emmet Sullivan, who had upbraided Flynn for his behaviour at a 2018 court appearance, signalled his skepticism at the government's unusual motion. He refused to dismiss the case and instead scheduled a hearing and appointed a retired federal judge to argue against the Justice Department's position. That former judge, John Gleeson, challenged the motives behind the department's dismissal request and called it a "gross abuse" of prosecutorial power.

Flynn's lawyers sought to bypass Sullivan and obtain an appeals court order that would have required the case's immediate dismissal. They argued that Sullivan had overstepped his bounds by scrutinizing a dismissal request that both sides, the defence and the Justice Department, were in agreement about and that the case was effectively moot once prosecutors decided to abandon it.

At issue before the court was whether Sullivan could be forced to grant the Justice Department's dismissal request without even holding a hearing into the basis for the motion.

"We have no trouble answering that question in the negative," the court wrote in an unsigned opinion for the eight judges in the majority.

The judges also rejected defence efforts to have the case reassigned to a different judge.

In response to the appeals court ruling, Sullivan on Tuesday directed the parties to file a joint status report by September 21 with a recommendation for future proceedings.

In a concurring opinion, Judge Thomas Griffith wrote that the court's opinion did not concern the merits of the Justice Department's prosecution of Flynn or even its decision to abandon the case. Rather, he said the question before the judges was a much more simple one.

"Today we reach the unexceptional yet important conclusion that a court of appeals should stay its hand and allow the district court to finish its work rather than hear a challenge to a decision not yet made," Griffith said. "That is a policy the federal courts have followed since the beginning of the Republic."

He said it was very possible that Sullivan could wind up granting the Justice Department's dismissal request and that it would in fact be "highly unusual if it did not, given the Executive's constitutional prerogative to direct and control prosecutions and the district court's limited discretion" in cases prosecutors want dropped.

Two judges, Neomi Rao and Karen LeCraft Henderson, each wrote dissenting opinions arguing that Sullivan had usurped his authority by keeping alive a case the Justice Department sought to have dismissed. Both judges were part of a 2-1 ruling in June that ordered Sullivan to dismiss the case.

"In Flynn's case, the prosecution no longer has a prosecutor," Rao wrote. "Yet the case continues with district court proceedings aimed at uncovering the internal deliberations of the Department. The majority gestures at the potential harms of such a judicial intrusion into the Executive Branch, but takes a wait-and-see approach, hoping and hinting that the district judge will not take the actions he clearly states he will take.

Flynn was questioned by the FBI just days after Trump's inauguration about his conversations with the then-Russian ambassador to the U.S. pertaining to sanctions that had just been imposed by the Obama administration for Russian election interference.

The private conversation alarmed law enforcement and intelligence officials who were already investigating whether the Trump campaign had co-ordinated with Russia to sway the presidential election in Trump's favour. Officials were also concerned by the White House's public insistence that Flynn and the diplomat had not discussed sanctions.

Flynn's guilty plea to lying to the FBI became a signature prosecution in special counsel Robert Mueller's investigation into ties between the 2016 Trump campaign and Russia. He also agreed to co-operate with the authorities in hopes of receiving a lighter sentence.

But as Flynn awaited sentencing, Attorney General William Barr appointed a U.S. attorney from St. Louis to investigate the handling of the Flynn case and later endorsed that prosecutor's recommendation that the case be dismissed.

In May, the Justice Department said it had concluded that the FBI had an insufficient basis to question Flynn about his conversations with the diplomat, which Barr has said were appropriate for an incoming national security adviser to have had, and that statements made during the interview were not material to the FBI's underlying counterintelligence investigation into the Trump campaign.

Law enforcement officials who were involved in the investigation vehemently disagreed with that conclusion.

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Associated Press writer Mark Sherman in Washington contributed to this report.