Democratic leaders on Capitol Hill are infuriated by reports that Donald Trump's Department of Justice is conducting a criminal investigation into the origins of the Russia probe.

They said the department was being used "to help the president with a political narrative for the next election".

House Intelligence Committee chair Adam Schiff and House Judiciary Committee chair Jerrold Nadler released a joint statement saying "profound new concerns" had been raised after it was reported on Thursday night that an administrative review into the matter had at some point switched into a criminal probe.

"If the Department of Justice may be used as a tool of political retribution or to help the president with a political narrative for the next election, the rule of law will suffer new and irreparable damage," their statement read.

It remains unclear what possible criminal actions are being investigated. The probe into Russian interference in the 2016 election did not find any criminal conspiracy between Russia and the Trump campaign.

But it did not clear the president of obstructing justice.

The reclassification allows John Durham, the US attorney spearheading the investigation under attorney-general William Barr, to assemble a grand jury, issue subpoenas and file criminal charges.

Mr Trump has long called for an investigation into the Russia probe led by former special counsel Robert Mueller, repeatedly describing it as a "witch hunt".

He has also claimed it was Ukraine that meddled in the 2016 US election in support of Hillary Clinton, rather than the Kremlin, which the US intelligence community has said worked in his favour.

Mr Trump urged Ukrainian president Volodymyr Zelensky to launch a probe into the origins of the US investigation into Russian meddling during a 25 July phone call that is now the subject of the House-led impeachment inquiry.

Mr Barr said he believed "spying did occur" against the president's 2016 campaign, during a Senate appropriations subcommittee hearing.

"The question is whether it was adequately predicated and I'm not suggesting it wasn't adequately predicated, but I need to explore that," he said in the April testimony. "I am going to be reviewing both the genesis and the conduct of intelligence activities directed at the Trump campaign during 2016."

He later said he was concerned about "improper surveillance" and added: "A lot of this has already been investigated and a substantial portion that's being investigated is being investigated by the Office of the Inspector General of the department."

The Department of Justice's Inspector General is also expected to release its own findings in a separate report that is "nearing completion", according to several outlets.

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