A senior staffer who advises the US Senate on Russia policy is under investigation for making trips to Ukraine in military uniform and supplying the country’s armed forces with privately donated sniper equipment, it was reported on Monday.

The actions of Kyle Parker, chief of staff to the Helsinki commission that informs senators on issues of European security, might have crossed legal and ethical grounds that could make him an unregistered foreign agent, according to the New York Times, which reviewed a confidential report by the commission’s director and general counsel.

Parker “traveled Ukraine’s frontlines wearing camouflage and Ukrainian military insignia and had hired a Ukrainian official for a US government fellowship over the objections of congressional ethics and security officials”, the newspaper said.

An independent law firm is investigating the allegations, and undertaking what the Times said was a broad investigation into conduct by staff of the commission, which has taken a strongly pro-Ukrainian stance as the country defends itself against the invasion by Russia.

The report, written by Helsinki commission executive director, Steven Schrage, and counsel Michael Geffroy, said Parker was “wittingly or unwittingly being targeted and exploited by a foreign intelligence service”, raising unidentified “counterintelligence issues” that needed to be reported to the FBI.

The Times said it was not known if federal law enforcement had been made aware. But it said Parker’s representatives insisted he had done nothing wrong and was instead the victim of retaliation by the report’s authors for making allegations of misconduct against them.

The Helsinki commission’s chairperson, South Carolina Republican congressman Joe Wilson, wanted Parker fired after he read the report, the Times said. “I urgently recommend you secure his immediate resignation or termination,” Wilson wrote to his co-chairperson, the Maryland Democrat Benjamin Cardin, citing “serious alleged improper acts involving Ukrainian and other foreign individuals”.

Parker is a longtime adviser to the commission and has traveled to Ukraine at least seven times since Russia invaded in February 2022, the Times said. He has delivered numerous pro-Ukraine lectures, podcasts and social media posts, and he describes himself as “the most well-traveled US official in Ukraine since” the war began, according to his online biography.

On one such visit, the report said, he handed over to Ukraine’s military about $30,000 of sniper’s range finders and ballistic wind gauges he had bought on Amazon in the US using money raised by a relative in Ukraine.

In a written response to the newspaper’s questions, a representative for Parker said none of his trips were official visits – even though the commission posted a photograph of him in Kherson – and that he never wore military insignia while dressed in army-style camouflage.

A video seen by the Times of Parker cutting up and urinating on a Russian hat was “a personal expression of rage and grief” at Russia’s brutality in Ukraine, the representative said.