Former Trump campaign manager Paul Manafort departs from U.S. District Court in Washington, U.S., February 28, 2018. (Xinhua/REUTERS)

WASHINGTON, Feb. 28 (Xinhua) -- Paul Manafort, former campaign chairman of U.S. President Donald Trump, pleaded not guilty on Wednesday to new charges levied against him in the probe into alleged Russian meddling in the 2016 election and will face trial in September.

Manafort's formal plea of not guilty came during a brief hearing in a Washington federal court regarding a second indictment accusing him of acting as an unregistered foreign agent and directing an international money-laundering conspiracy stemming from his political work in Ukraine.

It's Manafort's first court appearance since his longtime business associate Rick Gates, also a former Trump campaign aide, pleaded guilty and cut a deal with Special Counsel Robert Mueller, who is leading the Russia probe.

In court, Judge Amy Berman Jackson reprimanded Manafort for making a public statement about Gates' plea last week, saying the statement violated a gag order she put in place early in the case.

Jackson set the trial to start on Sept. 17 in Washington D.C. It is expected to stretch for several weeks and even through the U.S. midterm elections in early November.

The new charges filed Friday in Mueller's Russia probe said Manafort, with the help of Gates, used offshore accounts to wire more than 2 million euros (about 2.5 million U.S. dollars) to unnamed former senior European politicians in 2012 and 2013 on behalf of the then pro-Russia government of Ukraine.

File photo taken on July 21, 2016 shows U.S. Republican presidential nominee Donald Trump gives a thumbs up as his campaign manager Paul Manafort (C) and daughter Ivanka (R) look on during Trump's walk through at the Republican National Convention in Cleveland, the United States. (Xinhua/REUTERS)

The superseding indictment came hours after Gates pleaded guilty to charges of conspiracy against the United States and making false statements to the Federal Bureau of Investigation and pledged to cooperate with Mueller in the ongoing Russia probe.

In a statement prior to Friday's indictment, Manafort said he continued to maintain his "innocence" and that Gates' plea would not alter his commitment to defending himself against "the untrue piled up charges."

Manafort and Gates were originally indicted in October on charges of conspiracy and fraud stemming from their lucrative lobbying work in Ukraine, a case that arose out of Mueller's investigation into Russia's role in the election and possible collusion with the Trump campaign.

So far, none of the charges against them made reference to alleged Russia meddling nor accusations of collusion between Moscow and Trump' campaign.

Manafort alone was indicted in October on 12 counts related to unlawful financial dealings. The authorities at the time said Manafort laundered more than 18 million dollars.

Trump has vigorously denied allegations of collusion between his campaign and Moscow during the election. The Russian government has also repeatedly dismissed the accusations of interference.

So far, Mueller has charged nearly 20 people as part of his inquiry. Former Trump national security advisor Michael Flynn and former campaign foreign policy aide George Papadopoulos have each pleaded guilty to one count of lying.