Donald Trump's personal lawyer Rudy Giuliani with Ukrainian-American businessman Lev Parnas.

Democratic leaders of the House impeachment inquiry issued subpoenas on Thursday to two associates of President Donald Trump's lawyer Rudy Giuliani, who were arrested on charges of campaign finance violations overnight as they tried to flee the United States.

Lev Parnas, a Ukrainian-born American, and Igor Fruman, a Belarus-born US citizen, were arrested at Dulles Airport in Virginia on Wednesday evening as they sought to board an international flight with one-way tickets, Geoffrey Berman, US Attorney for the Southern District of New York, told a press conference.

The Florida-based businessmen "reportedly worked with Giuliani to pressure Ukraine for dirt on Trump's opponents," said Representative Adam Schiff, who is spearheading the impeachment inquiry.

The House is conducting an inquiry into Trump over alleged abuse of power for asking Ukrainian President Volodymr Zelensky in a July 25 phone call to conduct a probe of potential 2020 White House rival Joe Biden and his son, who served on the board of a Ukrainian gas company.

Schiff said Parnas and Fruman were ordered to turn "key documents" over to investigators, and to testify before the congressional panels.

"I don't know those gentlemen ... I don't know what they do," Trump said Thursday. "Maybe you will have to ask Rudy."

The subpoenas were issued within hours of the unsealing of an indictment charging Parnas and Fruman with campaign finance crimes.

The pair were accused of funneling foreign funds to US political campaigns, including $325,000 which went to a fundraising committee for Trump's 2020 reelection.

Foreign nationals are barred from contributing to US election campaigns.

According to the indictment, much of the funds came from "Foreign National-1," a Russian businessman seeking retail marijuana licenses in the United States.

"Foreign National-1" is alleged to have given Parnas and Fruman $1 million in two $500,000 installments, in September 2018 and October 2018.

The indictment said the illegal contributions by Parnas and Fruman were made "to advance their own personal financial interests" and the political interests of "at least one Ukrainian government official."

"The defendants concealed the scheme from the candidates, campaigns, federal regulators, and the public by entering into secret agreements, laundering foreign money through bank accounts in the names of limited liability corporations, and through the use of straw donors," it said.

$325,000 to Trump committee

Parnas and Fruman allegedly created a front company, Global Energy Producers (GEP), to funnel a $325,000 contribution to a fundraising committee identified as "Committee-1."

"Committee-1" is a pro-Trump fundraising committee known as America First Action, according to press reports.

The indictment does not make any specific mention of Giuliani but refers to efforts to remove the US ambassador to Ukraine, something the former New York mayor was involved in.

House Democrats are looking into whether the ambassador, Marie Yovanovitch, was removed early because she failed to go along with Giuliani's bid to get Ukraine to open an investigation into the Bidens.

Parnas and Fruman agreed to raise $20,000 or more for a US congressman to seek his "assistance in causing the US Government to remove or recall the then-US Ambassador to Ukraine," the indictment said.

"Parnas's efforts to remove the Ambassador were conducted, at least in part, at the request of one or more Ukrainian government officials," it said.

The name of the congressman was not given in the indictment but he is identified through campaign finance documents as former congressman Pete Sessions, a Republican from Texas.

Trump has openly admitted to seeking the removal of the ambassador, calling her "bad news."

Sessions said he did seek Yovanovitch's removal but it was because she was "disparaging President Trump to others." He also said he would "vigorously defend myself against any allegations of wrongdoing."

The House last month sent letters to Parnas and Fruman asking for testimony and documents related to their efforts to assist Giuliani in investigating the Bidens.

Their lawyer said they would not comply, in line with a White House decision to block all cooperation with the House impeachment inquiry.

In issuing the subpoenas, the House committee chairmen conducting the impeachment inquiry insisted that Parnas and Fruman testify.

"They are not exempted from this requirement merely because they happen to work with Mr. Giuliani and they may not defy congressional subpoenas merely because President Trump has chosen the path of denial, defiance and obstruction," they said.