Lebanese-American businessman George Nader, 60, entered a plea deal on Monday in federal court in Alexandria, Virginia.

Nader admitted transporting a 14-year-old boy from the Czech Republic to Washington DC in 2000 to engage in sexual activity with him. He also admitted possessing child pornography depicting infants or toddlers.

The pornography charge carries a 10-year mandatory minimum sentence. Prosecutors agreed as part of the plea bargain to recommend a 10-year sentence when Nader is sentenced in April, though the judge will be free to impose a significantly longer sentence if she chooses.

Nader's name appears more than 100 times in Mueller's report. It details Nader's efforts to serve as liaison between a Russian banker with ties to Russian President Vladimir Putin and members of President Donald Trump's transition team.

Nader is also under indictment in Washington along with seven others on charges of conspiring to conceal the source of more than $US3.5 million ($A5.1 million) in political donations to Hillary Clinton.

Nader previously served as an adviser to the United Arab Emirates, a close Saudi ally, and in 2017 wired $US2.5 million ($A3.6 million) to a top Trump fundraiser, Elliott Broidy, through a company in Canada, The Associated Press reported in 2018. The goal was to persuade the US to take a hard line against Qatar, a longtime American ally but now an adversary of the UAE.

Nader was previously convicted in the Czech Republic of 10 cases of sexually abusing minors and sentenced to a one-year prison term in 2003. The 14-year-old boy who was the victim in Monday's guilty plea is a different victim than the ones identified in the Czech case.

Nader also pleaded guilty to a charge of transporting child-pornography images in Virginia in 1991.

The current investigation of Nader began in 2018 when images depicting child pornography and bestiality were found on his phone after it was confiscated under a search warrant connected to the Mueller probe.

The images found in Nader's phones at Dulles International Airport ended up not being the basis for Monday's child-pornography conviction. Instead, prosecutors relied on images and videos he received via email in 2012 that in some cases involved sadistic depictions of infants or toddlers, according to disclosures at Monday's hearing.

In court papers filed on Monday, Nader admitted that he discussed child pornography online over the years with an unidentified associated, using the word "wine" as a substitute for child pornography to conceal the nature of the conversations. The two had also worked out a code to add 10 years to the age of persons they referred to in their conversations.

While Nader admitted to transporting the Czech boy to the US for the purpose of engaging in sexual activity, he argued that it happened too long ago to be prosecuted under the statute of limitations, and his plea deal preserves his right to appeal his conviction on that count on those grounds.

Even if such an appeal were granted, he would still be subject to a mandatory 10 years on the pornography charge.

Australian Associated Press