WASHINGTON — Paul J. Manafort, the former campaign chairman for President Trump who is at the center of investigations into Russian meddling in the 2016 presidential election, is working for allies of the leader of Iraq’s Kurdish region to help administer and promote a referendum on Kurdish independence from Iraq.

The United States opposes the referendum, but Mr. Manafort has carved out a long and lucrative career advising foreign clients whose interests have occasionally diverged from American foreign policy. And he has continued soliciting international business even as his past international work has become a focus of the investigation by the special counsel, Robert S. Mueller III, into ties between Russia and Mr. Trump and his associates, including possible collusion between them to influence the presidential election.

In fact, the work for the Kurdish group appears to have been initiated this summer around the time that federal authorities working for Mr. Mueller raided Mr. Manafort’s home in Virginia and informed him that they planned to indict him.

While the Kurdish referendum, scheduled for Monday, would not immediately trigger independence, the American government and the international community have expressed serious concerns about it. They fear that, if it passes overwhelmingly, as expected, it could further destabilize Iraq, damage the coalition fighting the Islamic State, and potentially spark violence in disputed areas.