Donald Trump lawyer Rudy Giuliani must register under the Foreign Agent Registration Act (FARA), experts say, after it was discovered he has maintained overseas clients while serving the president.

In May, the former New York mayor delivered a paid speech in Washington to Mujahideen-e-Khalq, or MEK, an Iranian resistance group formerly listed by the State Department as a terrorist organisation from 1997 to 2012, in which he called for regime change in Tehran. His consulting firm has also reportedly kept a list of clientele from cities across the globe, who have hired him to help with their security and police practices.

Such actions are considered "political activity" under FARA, according to Josh Rosenstein, an attorney with the firm Sandler Reiff Lamb Rosenstein & Birkenstock and an expert on the foreign agent statute. "The speech in Washington on behalf of MEK constituted measurable political activity under virtually any reading of the statute," he said. "What’s unusual is the fact that this administration seems to have one FARA violation after another … there is an emerging pattern of ties between the administration’s personnel and close friends — like Giuliani, Manafort and Gates — to foreign governments."

Mr Giuliani told the Washington Post it was unnecessary for him to register under FARA, saying, "I've never lobbied [Trump] on anything."

"I don’t represent foreign government in front of the US government," he continued. "I've never registered to lobby."

He also denied having to register as a foreign agent, since he works for the president pro-bono.

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Those defences are not included in any FARA exemptions, however. The only way Mr Giuliani could avoid registering under the statute would be if he had conducted the entirety of his work for international clients outside of the US while serving as the president’s lawyer since April, according to Mr Rosenstein.

Even then, his speech to MEK would likely require him to register under FARA, according to guidelines for the statute put out by the Justice Department.

"Delivering speeches to audiences in the United States may very well be treated as political activity under FARA, and that’s because the statute includes political activity to influence the US public," he said. "And it makes no difference if he’s working for the president for free; while there are exemptions to the law, who your other clients are are totally irrelevant."

Despite his claims that he’s never lobbied the president “on anything,” reports indicate Mr Trump’s attorney has pushed for him to promote his son, who works at the White House Public Liaison’s Office. The White House did not respond to a request for comment on whether the president has spoken with his outside legal counsel about political matters.

Still, apparent connections between some of Mr Giuliani’s international clientele and the president's international political matters represent clear conflicts of interest, according to legal experts.

Mr Giuliani has worked for the Ukrainian city of Kharkiv this year, and travelled to the country in 2017 to meet with Mayor Hennadiy Kernes.

Mr Giuliani is expected to continue his work with the Ukrainian mayor later this year. He also told the Washington Post that over the last ten years he’s received regular payments from MEK, which he currently maintains as a client.

The recent developments have sparked backlash from both national security and legal experts. "An unregistered foreign agent is serving as the president’s primary personal lawyer in the Russia probe," Bradley Moss, a national security lawyer and media commentator, wrote on Twitter. "The fish stinks from the head."

"I think Rudy believes because he is doing the job pro bono the rules do not apply to him, but they do," Carrie Menkel-Meadow, a professor at University of California-Irvine and legal ethics expert, told the Washington Post. Ms Menkel-Meadows said it's uncommon for a president’s outside legal counsel to keep foreign clientele due to competing interests.

Despite an increasing effort since 2016 to enforce FARA at a federal level, prosecutions for violations remain rare and enforcement of the statute is weak. According to a 2016 report, the Justice Department only made seven criminal cases regarding FARA from 1996 to 2015, and the majority of filings were considered late.

However, several recent FARA-related charges have been made against Mr Trump’s inside circle, including Mr Manafort and former Trump campaign official Rick Gates.

While FARA does not include specific violations for conflicts of interest, such an issue could potentially jeopardide Mr Giuliani’s admission as an attorney to the New York State bar.

"If any attorney is representing two clients with diverging interests, those conflicts may be waived but there has to be notice and consent from both sides," he said. “There are certain conflicts that cannot be waived, though it remains to be seen whether there are any conflicts and how great they are."