Rudy Giuliani, President Donald Trump's personal attorney, called Venezuelan leader Nicolas Maduro late last year in an attempt to oust him from the presidency, The Washington Post reported, citing people familiar with the endeavor.

The call is another instance of Giuliani's involvement in U.S. diplomatic policy. The former New York mayor has come under intense scrutiny for his continuing efforts to dig up dirt on former Vice President Joe Biden and his son Hunter in Ukraine.

Trump's request for Ukraine President Volodymyr Zelenskiy to announce investigations into the Bidens, along with a conspiracy theory about Ukraine meddling in the 2016 presidential election, are at the center of the charges in the president's impeachment in Congress.

In September 2018, Giuliani and former Rep. Pete Sessions, R-Texas, called Maduro as part of a back-channel effort to broker a negotiated exit for the embattled Venezuelan leader, according to the Post.

Sessions' spokesman Matt Mackowiak confirmed the Post's report to CNBC.

"Congressman Sessions traveled to Caracas in April 2018 to conduct direct negotiations with President Maduro on a diplomatic mission that was coordinated by the highest levels of the State Department," Mackowiak said in an emailed statement.

"The goal was to arrange Maduro's peaceful exit to allow a transition to a democratically elected government. Several months later a conference call was held with several participants, including Congressman Sessions and Mayor Giuliani," he said.

People familiar with the State Department told the Post that officials there did not organize or participate in Sessions' communications with Maduro. The newspaper reported that several U.S. officials disputed that the government backed Sessions' trip.

The White House declined CNBC's request for comment on the Post's report.

The U.S. has challenged Maduro's legitimacy to lead Venezuela and has joined an international coalition pressuring him to cede power to Juan Guaido, whom they have recognized as the country's acting president.

The call was made without approval from White House officials, who did not know why Giuliani was involved once they found out about his actions, one former senior administration official told the Post.

The back-channel attempt to deal with Maduro defied the official U.S. policy led by then-national security advisor John Bolton, which involved a campaign of tightening sanctions on the Venezuelan regime, the newspaper reported. Giuliani's Ukraine involvement also was outside of official U.S. policy.

Lev Parnas, an associate of Giuliani who played a role in his Ukraine dealings, was also involved in the shadow diplomacy effort with Venezuela, according to the Post.

Mackowiak told the Post that Sessions met Parnas on Capitol Hill in May, and that Parnas wanted to talk about a proposal to sell liquefied natural gas in Ukraine. That meeting came about five weeks after Sessions traveled to Caracas intending to spur progress in Venezuela, according to his comments to the Dallas Morning News in 2018.

In August 2018, Parnas met with Giuliani and two American executives with business investments in Venezuela to talk about the back channel to Maduro, the Post reported.

Parnas was arrested in October on charges of violating federal campaign finance laws. His indictment alleges that he and another associate made illegal donations to a then-sitting U.S. congressman, speculated to be Sessions, and that Parnas had asked that congressman for assistance in getting the U.S. to remove its ambassador to Ukraine.

Sessions sent Secretary of State Mike Pompeo a letter urging that ambassador's removal on May 9 — the same day Parnas posted photos of his meeting with Sessions on social media, the Post reported.

Mackowiak told the Post that Sessions did not act at Parnas' request, but rather wrote the letter after hearing concerns about the ambassador from other members of Congress.

Last month, Parnas' lawyer Joseph Bondy said Parnas was willing to testify before Congress as part of an ongoing impeachment proceeding against Trump.

Read the Post's full report here.