A senior Capitol Hill staff member who is a longtime voice on Russia policy is under congressional investigation over his frequent trips to Ukraine’s war zones and providing what he said was $30,000 in sniper gear to its military, documents show.

The staff member, Kyle Parker, is the senior Senate adviser for the U.S. Commission on Security and Cooperation in Europe, known as the Helsinki Commission. The commission is led by members of Congress and staffed by congressional aides. It is influential on matters of democracy and security and has been vocal in supporting Ukraine.

A confidential report by the commission’s director and general counsel, which The New York Times reviewed, said that the equipment transfer could make Mr. Parker an unregistered foreign agent. It said that Mr. Parker had traveled Ukraine’s front lines wearing camouflage and Ukrainian military insignia and had hired a Ukrainian official for a U.S. government fellowship over the objections of congressional ethics and security officials.

And it raised the possibility that he was “wittingly or unwittingly being targeted and exploited by a foreign intelligence service,” citing unspecified “counterintelligence issues” that should be referred to the F.B.I.

A representative for Mr. Parker said he had done nothing wrong. He said Mr. Parker was the target of a “campaign of retaliation” for making accusations of misconduct against the report’s authors.