Two foreign-born Florida businessmen who helped US President Donald Trump's personal lawyer Rudy Giuliani investigate political rival Joe Biden have pleaded not guilty to illegally funnelling money to a pro-Trump election committee and other politicians.

Ukraine-born Lev Parnas and Belarus-born Igor Fruman entered their not guilty pleas to federal campaign finance charges before US District Judge Paul Oetken in Manhattan on Wednesday.

Federal prosecutors have accused Parnas and Fruman of using a shell company to donate $US325,000 to the pro-Trump committee, America First Action, and raising money for former Texas congressman Pete Sessions as part of an effort to have the Republican president remove the US ambassador to Ukraine.

Parnas and Fruman's arrest at a Washington-area airport earlier this month, as they prepared to fly out of the US on one-way tickets, was yet another political hazard for Trump amid a fast-moving impeachment inquiry by the Democratic-led US House of Representatives.

Both Parnas and Fruman are free on bail.

The two men have been charged with conspiracy, making false statements to authorities and falsifying business records.

Joseph Bondy, a lawyer for Parnas, told reporters outside the courthouse he looked forward to proceeding based on the evidence, not "a smear campaign that's been driven by self serving and misleading leaks apparently from the highest levels of our government".

Parnas himself also denied the charges against him.

"I will be vindicated," he said.

Prosecutors are examining Giuliani's interactions with the two men, according to a source familiar with the matter.

Giuliani has said Parnas and Fruman helped his efforts in Ukraine to investigate Biden and Biden's son Hunter. The younger Biden had served as a director of a Ukrainian energy company.

Giuliani has denied wrongdoing.

Prosecutors said Parnas and Fruman funnelled money from an unnamed Russian businessman to political candidates in several states to help the businessman obtain permits needed for a proposed marijuana business, which never materialised. US law prohibits foreign donations to political campaigns.

Prosecutors say Parnas and Fruman committed to raise $US20,000 for Sessions as part of an effort to get Trump to remove US ambassador to Ukraine Marie Yovanovitch.

That effort was carried out at the request of at least one Ukrainian official, prosecutors said.

Trump ordered Yovanovitch removed in May. Yovanovitch testified in the House impeachment inquiry on October 11.

Australian Associated Press